Musannaf Ibn Abi Shayba 9:1461

Ibn ‘Ulayyah narrated to us, from Ayyub, who said: The first time I heard of "the milk of the stallion" [i.e., milk resulting from a pregnancy caused by a specific man] was while we were in Makkah. Iyas ibn Mu’awiyah began saying: "And what is the harm in this, and who dislikes this?"