John Cassian, On the Incarnation of the Lord, Against Nestorius

But indeed in your deceit and blasphemy you use a grand argument for denying and attacking the Lord God, when you say that “the child born ought to be of one substance with the mother.”204 Homoousios parienti debet esse nativitas. I do not entirely admit it, and maintain that in the matter of the birth of God it would not be observed; for the birth was not so much the work of her who bore Him as of her Son, and He was born as He willed, whose doing it was that He was born. Next, if you say that the child born ought to be of one substance with the parent, I affirm that the Lord Jesus Christ was of one substance with His Father, and also with His mother. For in accordance with the difference of the Persons He showed a likeness to each parent. For according to His Divinity He was of one substance with the Father: but according to the flesh He was of one substance with His mother. Not that it was one Person who was of one substance with the Father, and another who was of one substance with His mother, but because the same Lord Jesus Christ, both born as man, and also being God, had in Him the properties of each parent, and in that He was man He showed a likeness to His human mother, and in that He was God He possessed the very nature of God the Father.