Muwatta Imam Malik (Riwayat Yahya)

Imam Malik, may Allah have mercy on him, said: This is because Allah, the Glorious and Exalted, has said: "Your mothers are forbidden to you, and your wives' [mothers]." So Allah has made mothers unlawful due to marriage with their daughters, not due to fornication. Therefore, when marriage is contracted with a woman, even if it is invalid, the prohibition is established, but not through fornication.