Maliks Muwatta Book 28, Hadith Number 22.

Section : Prohibition against Marrying Mothers of Wives.

Yahya related to me from Malik from Yahya ibn Said that Zayd ibn Thabit asked whether it was halal for a man who married a woman and then separated from her before he had cohabited with her, to marry her mother. Zayd ibn Thabit said, “No. The mother is prohibited unconditionally. There are conditions, however about foster-mothers.”