Maliks Muwatta Book 030, Hadith Number 005.

Section : Suckling of the Young.

Yahya related to me from Malik from Ibn Shihab from Amr ibn ash-Sharid that Abdullah ibn Abbas was asked whether, if a man had two wives, and one of them nursed a slave-boy, and the other had nursed a slave-girl, could the slave-boy marry the slave-girl. He said, "No. The husband is the same."