Where did Thomas Aquinas get this quote from?

The citation given ... is ... "S.Th. ia 2ae 96. 5." I understand "S.Th." to refer to the Summa Theologica, although I do not understand what the rest of this citation signifies.

• It's actually "1a" and not "ia" (what looks like a small caps ɪ is actually just the Hindu-Arabic numeral 1).

• "1a 2ae" = "Prima Secundae" (or the "First part of the Second Part"). (Sometimes also given as "Ia IIae".)

• "96" = Question 96.

• "5" = Article 5.

We can then look up S.Th. 1a 2ae 96. 5 at e.g. Sources 1, 2, 3, or 4.

in what part of the Summa Theologica does Thomas Aquinas quote this passage?

We must distinguish between two parts of Kelly's quote; I'll call them Parts A and B (now reproduced):

A. And in the Codex of Justinian the emperors Theodosius and Valentinian write to the prefect Volusianus: 'It is a saying worthy of the majesty of a ruler, if the prince professes himself bound by the laws: for even our authority depends upon that of the law. And, in fact, the most important thing in government is that power should be subject to laws'

B. ... [And] in the judgment of God, a ruler is not free from the directive power of the law; but should voluntarily and without constraint fulfil it.

Part B does indeed appear in Summa at the cited "S.Th. 1a 2ae 96. 5". From e.g. Source 1:

Hence, in the judgment of God, the sovereign is not exempt from the law, as to its directive force; but he should fulfil it to his own free-will and not of constraint.

In contrast, Part A does not appear in any of Sources 1, 2, 3, or 4.

where does this passage come from originally?

We've already dealt with Part B (it comes directly from Aquinas).

Now, Part A comes originally from the Code of Justinian (effective 529), Book I, Title 14, Law/Constitution 4 (this is often abbreviated as "CI. 1.14.4" where "CI" stands for Codex Iustinianus).

From a 1932 translation by S. P. Scott (as reproduced at this webpage):

4. The Same Emperors to Volusianus, Praetorian Prefect.

It is a statement worthy of the majesty of a reigning prince for him to profess to be subject to the laws; for Our authority is dependent upon that of the law. And, indeed, it is the greatest attribute of imperial power for the sovereign to be subject to the laws and We forbid to others what We do not suffer Ourselves to do by the terms of the present Edict.

Given at Ravenna, on the third of the Ides of June, during the Consulate of Florentinus and Dionysius, 429.

^{Note: "the third of the Ides of June" = June 11th.}

Next, in Aquinas: Selected Political Writings (1948 translation by John G. Dawson of Alessandro Passerin d'Entrèves's 1946 Scritti politici [di] San Tommaso d'Aquino), we find the following passage (at pp. 139, 141; links here to a 1965 printing), which is almost† exactly identical to the above passage by Kelly that you quoted:

And in the Codex, the Emperors, Theodosius and Valentinian, write to the Prefect Volusianus: 'It is a saying worthy of the majesty of a ruler, if the prince professes himself bound by the laws: for even our authority depends upon that of the law. And, in fact, the most important thing in government is that power should be subject to laws.' ... So, in the judgement of God, a ruler is not free from the directive power of the law; but should voluntarily and without constraint fulfil it

^{†The only differences are these: Kelly adds "of Justinian", decapitalizes "Emperors" and deletes a comma after, omits a comma after "Valentinian", decapitalizes "Prefect", adds "[And]" in brackets, and changes the spelling "judgement" to "judgment".}

So, instead of explicitly telling the reader that he copied (with some errors) the above passage (in a 1948 English translation), Kelly simply cites "S.Th. 1a 2ae 96. 5" as if he used the original source.

Now, in the 1948 Translator's Note (p. xxxv), we read:

The editions from which the Latin text is taken are the Leonine (Rome, 1889-1918 ss.) for the passages from the Summa Theologica

This refers to the Editio Leonina "originally sponsored by Pope Leo XIII in 1879." So, when we actually go to this edition and look up "1a 2ae 96. 5" in Tomus Septimus, Prima Secundae Summa Theologiae (1892, pp. 184f), we find that Part A (of Kelly's quote) actually appears only in this footnote:

Aliquae editiones addunt: Et in Codice, Theodosius et Valentinianus, Imperatores, Volusiano Praefecto scribunt: Digna vox est maiestate regnantis legibus alligatum se principem profiteri: adeo de auctoritate iuris nostra pendet auctoritas. Et re vera maius imperio est subiicere legibus principatum

Google Translation:

Some editions add: And in the Codex, Theodosius and Valentinianus, the emperors, write to the Volusian prefect: It is a worthy voice for the majesty of a ruler bound by the laws to declare himself a prince: so much does authority depend on the authority of our law. And in truth it is a greater power to submit to the laws of the principality

Unfortunately, the 1948 English translation (and possibly also the 1946 Italian text) seems to omit the important piece of information that the above lines appear only in "Some editions". Instead, it simply states the above "Emperors ... write to the Prefect Volusianus" lines (Part A quote) with seemingly complete confidence that Aquinas actually wrote these lines. (To be fair, this Selected Political Writings is meant to be a somewhat concise book, ending on p. 199.)

Kelly then copies the 1948 English translation without proper citation.

So, we know where Part A is originally from. But we don't know whether Aquinas actually had (some version of) Part A in his Summa.

Unfortunately, academics (both now and in medieval times) are too often careless in citing their sources and quoting others exactly and correctly (we've seen this here with Kelly and also the 1948 English translation).

My guess is this: Quite possibly, at some point, some edition of Summa added the above Part A passage to some edition of Summa Theologica, perhaps initially only as some footnote or sidenote commentary/interesting additional information. Then later, some other editions started mistakenly attributing this passage to Aquinas outright.

The first part of the question is: what is the original text by Thomas?

The problem is that there have been a huge number of editions through time. Some of them have the sentence, some not. I found it in the

Summa theologica. Editio altera romana ad emendatiores editiones impressa et noviter accuratissime recognita, t. 2: Prima Secundae Partis (Ex typographia Forzani, Romae, 1894).

It is in the first part of the second book (Prima Pars Secundae Partis), Quaestio 96: "Deinde considerandum est de potestate legis humanae." Article 5, "Utrum omnes subjiciantur leges" ends with this final paragraph:

Ad tertium dicendum, quod princeps dicitur esse solutus a lege quantum ad vim coactivam legis: nullus enim proprie cogitur a seipso: lex autem non habet vim coactivam, nisi ex principis potestate; sic igitur princeps dicitur esse solutus a lege, quia nullus in ipsum potest judicium condemnationis ferre, si contra legem agat; unde super illud Psal. 50.: Tibi soli peccavi, etc. dicit Gloss. (ord. Cassiod.), quod rex non habet hominem, qui sua facta dijudicet: sed quantum ad vim directivam legis princeps subditur legi propria voluntate: secundum quod dicitur extra de constitutionibus, cap. Cum omnes: Quod quisque juris in alterum statuit, ipse eodem juri uti debet. Et Sapientis dicit auctoritas (Cat. in rudiment.): Patere legem, quam ipse tuleris; et in Codice (lib. 4. de Legib. et Constitut.) Theodosius, et Valentinianus Impp. Volusiano Praefecto scribunt: Digna vox est majestate Regnantis, legibus alligatum se principem profiteri, adeo de auctoritate juris nostra pendet auctoritas: et revera majus imperio est subjicere legibus principatum. Improperatur etiam his a Domino, qui dicunt, et non faciunt; et qui aliis onera gravia imponunt, et ipsi nec digito volunt ea movere, ut dicitur Matth. 23.; unde quantum ad Dei judicium princeps non est solutus a lege quantum ad vim directivam ejus; sed debet voluntarius, non coactus, legem implere. Est etiam princeps supra legem, inquantum, si expediens fuerit, potest legem mutare, et in ea dispensare pro loco, et tempore.

The sentence in question is only part of that edition, in other editions, even those of the so-called Leonine edition, it is simply missing. The 1882 edition has moved the sentence simply to a footnote with the notice "Aliquae editiones addunt:...".

So far I have not found critical editions that really explain the difference. From the history of publication it is clear the "Editio altera" represents an older tradition, while the "Leonine" editions are an attempt to produce critical texts – a method quite new at the time. The edition of Thomas' collected works was commissioned by Pope Leo XIII and published by the Master General of the Dominicans under the direction of a commission of Cardinals. The text of Book II was produced by comparing nine manuscripts from the Vatican library with the first edition published by the Domenican order at its Santa Maria sopra Minerva school), the "Piana" (1570, after Pope Pius V.), and documenting the variants. The way the footnote is formulated, it seems that the sentence in question was not part of any of them.

That might indicate that the editors were quite certain it was not by Thomas, but a case of

marginal notes...which, over time, were added to the text due to the inexperience of the scribes and were thereafter considered as if they were parts of the text itself...

However, the varieties of readings, which are of absolutely no importance... and others of that kind which occur here and there, I have either omitted or used, without adding any annotation; But other readings which seemed of some importance, I either indicate in the margin or add in the notes, and assign a reason why I have adopted one over the others, and if I refrain from adopting some, I nevertheless transcribe them and submit them to the judgment of wiser men...

_{Introduction to vol. I of the Leonine edition, p. XXXVII. English translation from Latin by Google translate}

But it needs to be said that the Leonine volumes containing the Summa have been criticised for a certain lack of scientific "neutrality". While this argument was refused by the Domenican editors, it remains true that pressure from the Pope to produce results lead to some haste in publishing the volumes, and more thorough explanations of the way the text was achieved are missing. See Concetta Luna, L'édition léonine de saint Thomas d'Aquin, Revue des sciences philosophiques et théologiques 2005/1 (Tome 89), p. 31–110 for more on the publication history.

The second sentence in bold is the one Kelly quotes as his last sentence (starting with "in the judgment of God"). It is not a part of the Theodosian law, but the commentary of Thomas.

The second part of the question, where does the original quote come from? seems to have a simple answer:

in Codice (lib. 4. de Legib. et Constitut.) Theodosius, et Valentinianus Impp. Volusiano Praefecto scribunt

This seems to refer to the Codex Theodosianus. But that is not the case. Instead, the paragraph can be found in the Codex Iustinianis, Book One, 14th title, no. 4. I am citing both the Latin text and the English translation from Bruce W. Frier, Fred H. Blume, The Codex of Justinian: A New Annotated Translation with Parallel Latin and Greek Text, Cambridge 2016, pp. 258–261.

Idem AA. [Impp. Theodosius et Valentinianus] ad Volusianum pp. Digna vox maiestate regnantis legibus alligatum se principem profiteri: adeo de auctoritate iuris nostra pendet auctoritas, et re vera maius imperio est submittere legibus principatum, et oraculo praesentis edicti quod nobis licere non patimur indicamus.
D. in id, lun. Ravennae Florentia et Dionysio conss.

The same Augusti [Emperors Theodosius and Valentinian] to Volusianus, Praetorian Prefect. It is a statement worthy of the majesty of a ruler to proclaim himself bound by the laws: so much does Our authority depend on the authority of the law. And in truth, to subject the sovereign power to the laws is something greater than imperial rule. And by the oracular pronouncement of the present edict, We show what We do not allow Ourselves.
Given June 11, at Ravenna, in the consulship of Florentius and Dionysius (429).