The proof from Cyril of Alexandria:
For we believe in one God, Father almighty, maker of all things both visible and invisible, and in one Lord Jesus Christ, and in the Holy Spirit; and following the professions of faith of the holy fathers that supplement this, we say that the Word begotten essentially from God the Father became as we are and took flesh and became man, that is, he took for himself a body from the holy Virgin and made it his own. For that is how he will truly be one Lord Jesus Christ, that is how we worship him as one, (not separating man and God), but believing that he is one and the same in his divinity and his humanity, that is to say, (simultaneously both God and man.)-Against Nestorius(pg.141)
For (the incarnate nature of the Word is immediately conceived of as one after the union).30 It is not unreasonable to see something similar in our own case too. For (a human being is truly one compounded of dissimilar elements), by which I mean soul and body. But it is necessary to note here that we say that the body united to God the Word is endowed with a rational soul. And it will also be useful to add the following: (the flesh, by the principle of its own nature, is different from the Word of God, and conversely the nature of the Word is essentially different from the flesh). Yet even though the elements just named are conceived of as different and separated into a dissimilarity of natures, (Christ is nevertheless conceived of as one from both, the divinity and humanity having come together in a true union, Against Nestorius in Russell's Cyril of Alexandria, pg.142)