This is from Tertullian's work Adversus Praxean (27):

The Word, therefore, is incarnate; and this must be the point of our inquiry: How the Word became flesh,—whether it was by having been transfigured, as it were, in the flesh, or by having really clothed Himself in flesh. Certainly it was by a real clothing of Himself in flesh. For the rest, we must needs believe God to be unchangeable, and incapable of form, as being eternal. But transfiguration is the destruction of that which previously existed. For whatsoever is transfigured into some other thing ceases to be that which it had been, and begins to be that which it previously was not.

God, however, neither ceases to be what He was, nor can He be any other thing than what He is. The Word is God, and “the Word of the Lord remaineth for ever,”—even by holding on unchangeably in His own proper form. Now, if He admits not of being transfigured, it must follow that He be understood in this sense to have become flesh, when He comes to be in the flesh, and is manifested, and is seen, and is handled by means of the flesh; since all the other points likewise require to be thus understood. For if the Word became flesh by a transfiguration and change of substance, it follows at once that Jesus must be a substance compounded of two substances—of flesh and spirit,—a kind of mixture, like electrum, composed of gold and silver; and it begins to be neither gold (that is to say, spirit) nor silver (that is to say, flesh),—the one being changed by the other, and a third substance produced.

Jesus, therefore, cannot at this rate be God for He has ceased to be the Word, which was made flesh; nor can He be Man incarnate for He is not properly flesh, and it was flesh which the Word became. Being compounded, therefore, of both, He actually is neither; He is rather some third substance, very different from either. But the truth is, we find that He is expressly set forth as both God and Man; the very psalm which we have quoted intimating (of the flesh), that “God became Man in the midst of it, He therefore established it by the will of the Father,”—certainly in all respects as the Son of God and the Son of Man, being God and Man, differing no doubt according to each substance in its own especial property, inasmuch as the Word is nothing else but God, and the flesh nothing else but Man.

Thus does the apostle also teach respecting His two substances, saying, “who was made of the seed of David;” in which words He will be Man and Son of Man. “Who was declared to be the Son of God, according to the Spirit;” in which words He will be God, and the Word—the Son of God. We see plainly the twofold state, which is not confounded, but conjoined in One Person—Jesus, God and Man.