Nazianzen to Eunomius on Begotten
Thus in reply to Eunomius [Gregory Nazianzen] says, "If you say that he who begat and he who is begotten are not the same, the statement is incorrect. For it is in fact a necessary truth that they are the same. For the nature of the relation of father to child is this, that the offspring is of the same nature as the parent."
Kevin Giles, The Eternal Generation of the Son: Maintaining Orthodoxy in Trinitarian Theology (Downers Grove, IL: InterVarsity Press, 2012), 0