Maliks Muwatta Book 28, Hadith Number 21.

Section : Combinations of Women Not to be Married Together.

Yahya related to me from Malik from Yahya ibn Said that Said ibn al-Musayyab said, “It is forbidden to be married to a woman and her paternal or maternal aunt at the same time, and for a man to have intercourse with a female slave who is carrying another man’s child.”