We all know that Ockham's razor wasn't really Ockham's razor. He got the shaving device second hand from his predecessors, among them, Aquinas.
Below are some instances of Aquinas's use of it, which I have shamelessly lifted from Schütz's Lexikon. Schütz lists them in the entry for fieri (and you'll see why). I came across them last week and I thought it would be handy to gather them here for anyone who is interested in the topic.
Three things to note: (1) Of the instances from the Contra gentiles and the Summa theologiae below (which are all the instances save one), almost all are found in objections. The only one that isn't from an objection is the one from CG, I, 42 (the first one). (2) The instance from the commentary on the Physics (the last one) is used in explicating Aristotle's argument. (3) I made minor changes to the wording and punctuation of the second and last ones since I noticed discrepancies with the Leonine text.
Don't cut yourself!
quod sufficienter fit uno posito, melius est per unum fieri, quam per multa (CG, I, 42)
quod potest sufficienter fieri per unum, superfluum est si per multa fiat (CG, III, 70)
quod potest compleri per pauciora principia, non fit per plura (ST, I, a. 2, arg. 2)
quod potest sufficienter fieri per unum, superfluum est, quod fiat per multa (ST, I, q. 108, a. 3, arg. 2)
quod sufficienter potest fieri per unum, non oportet, quod per aliquid aliud inducatur (ST, II-II, q. 22, q. 1, arg. 1)
quod potest fieri per unum, superfluum est plura ponere (ST, II-II, q. 45, a. 2, arg. 3)
quod potest fieri per unum, superflue fit per multos (ST, III, q. 82, a. 2, arg. 2)
Quod potest fieri per pauciora, superfluum est si fiat per plura (In Physic., I, l. 11, n. 14)