Musannaf Ibn Abi Shayba 9:1445

Abu Bakr narrated to us, he said: Idris narrated to us, from Ibn Jurayj, from ‘Amr ibn al-Sharid, he said: Ibn ‘Abbas was asked about a man who had a wife and a slave-concubine, and one of them breastfed a boy and the other breastfed a girl. Is it permissible for the boy to marry the girl? He said: "No, the source of the milk is one."