The prophecy was in the Sibylline Books. For the prophecy itself, see Suetonius Iuius 79; Dio Cassius 44.15.3, Appian BC 2.110, Plutarch Caes 60. Cicero brings it up as part of a larger discussion of prophecies and the "Noble Lie" in his treatise de Divitione book 2, beginning at chapter 110. He does not mention the specific prophecy at all, though, and the mockery (if you can call it that) comes via the discourse of one of his interlocutors:

"But what weight is to be given to that frenzy of yours, which you term 'divine' and which enables the crazy man to see what the wise man does not see, and invests the man who has lost human intelligence with the intelligence of the gods? We Romans venerate the verses of the Sibyl who is said to have uttered them while in a frenzy. Recently there was a rumour, which was believed at the time, but turned out to be false, that one of the interpreters​ of those verses was going to declare in the Senate that, for our safety, the man whom we had as king in fact should be made king also in name. If this is in the books, to what man and to what time does it refer? For it was clever in the author to take care that whatever happened should appear foretold because all reference to persons or time had been omitted. link

For a full discussion of this passage, see Krostenko, "Beyond (Dis)belief: Rhetorical Form and Religious Symbol in Cicero's de Divinatione" TAPA 130 (2000): 353-391.