Al-Muwatta Hadith 28.23

Yahya related to me from Malik from more than one source that when Abdullah ibn Masud was in Kufa, he was asked for an opinion about marrying the mother after marrying the daughter when the marriage with the daughter had not been consummated. He permitted it. When Ibn Masud came to Madina, he asked about it and was told that it was not as he had said, and that this condition referred to foster-mothers. Ibn Masud returnedto Kufa,and he had just reached his dwelling when the man who had asked him for the opinion came to visit and he ordered him to separate from his wife. Malik said that if a man married the mother of a woman who was his wife and he had sexual relations with the mother then his wife was haram for him, and he had to separate from both of them. They were both haram to him forever, if he had had sexual relations with the mother. If he had not had relations with the mcther, his wife was not haram for him, and he separated from the mother. Malik explained further about the man who married a woman, and then married her mother and cohabited with her, "The mother will never be halal for him, and she is not halal for his father or his son, and any daughters of hers are not halal for him and so his wife is haram for him." Malik said, "Fornication however, does not make any of that haram because Allah, the Blessed, the Exalted, mentioned 'the mothers of your wives,' as one whom marriage made haram, and he didn't mention the making haram by fornication. Every marriage in a halal manner in which a man cohabits with his wife, is a halal marriage. This is what I have heard, and this is how things are done among us."