The passage can be found in Book 4 paragraph 33.5 of St. Augustine commentary on the Heptateuch (first seven books of the Hebrew Bible: Genesis to Judges) commonly referred to as Quaestiones in heptateuchum VII (Seven Questions Concerning the Heptateuch). Book 4 is his commentary on the Book of Numbers.
The English translation of Seven Questions Concerning the Heptateuch is part of Volume 1/14 (Writings on the Old Testament) of "Works of Saint Augustine: A translation for the 21st Century" which can be purchased from New City Press (hard copy) or from Logos (digital version). The translation is done by Joseph T. Lienhard, S.J. From the publisher blurb:
In 1990, New City Press, in conjunction with the Augustinian Heritage Institute, began the project knows as The Works of Saint Augustine, A Translation for the 21st Century. The plan is to translate and publish all 132 works of Saint Augustine, his entire corpus, into modern English. This represents the first time in which the works of Saint Augustine will all be translated into English. Many existing translations were often archaic or faulty, and the scholarship was outdated. The Works of Saint Augustine, A Translation for the 21st Century will be translated into 49 published books. To date, the Complete Set includes 44 containing 93 of Augustine's works.
The Latin passage in question (Book 4 paragraph 33.5) is St. Augustine's commentary on Numbers 19:5 (ESV):
And the heifer shall be burned in his sight. Its skin, its flesh, and its blood, with its dung, shall be burned.
Numbers 19:5 in Latin (Vulgate):
comburetque eam cunctis videntibus, tam pelle et carnibus ejus quam sanguine et fimo flammae traditis.
The complete Latin of Quaestiones in heptateuchum VII can be found here (click "Liber 4" for the Book of Numbers). Here's the quote you're looking for with italic text referring to the phrases he commented on:
Et cremabunt eam in conspectu eius. Puto quia concrematio ad signum pertinet resurrectionis. Natura est quippe ignis ut in superna moveatur, et in eum convertitur quod crematur. Nam et ipsum cremare de graeco in latinum ductum verbum est a suspensione. Quod vero additum est, in conspectu eius, id est in conspectu sacerdotis, hoc mihi insinuatum videtur, quia illis apparuit resurrectio Christi, qui futuri erant regale sacerdotium. Iam quod sequitur: Et pellis eius et carnes et sanguis eius cum stercore eius comburetur, id ipsum expositum est quomodo concremabitur: et significatum est quod non solum substantia mortalis corporis Christi, quae commemoratione pellis et carnium et sanguinis intimata est; verum etiam contumelia et abiectio plebis, quam nomine stercoris significatam puto, converteretur in gloriam, quam combustionis flamma significat.
I cannot yet find a freely available English translation online (will update this answer if I can find it), but the Google Translate result should give you some idea:
He burned it in front of him. I think that the signal belongs to the burning of the resurrection. Since the nature of the fire to move higher, and in turn it is burned. It is far different from the Greek word is derived from the floor. But what is added is to say, in the sight of her, that is, in the sight of the priest, is this to me was insinuated into the account, that he appeared to them came also the resurrection of Christ, who were going to be a royal priesthood. Now as to what there follows, And its skin and its flesh, and its dung, shall be burned up, and the blood was made like, that in itself has been explained in what way they concremabitur: mortal, and it hath been signified that it is not only the substance of the body of Christ, which is notified to the commemoration of the skin, and with the flesh and of the blood; but also the insult and the outcast of the people, as in the name signified 'dung, I think, would be changed into the glory, which is signified by the burning of a flame.