Maliks Muwatta Book 29, Hadith Number 20.
Section : Dhihar of Free Men.
Yahya related to me from Malik from Said ibn Amr ibn Sulaym az-Zuraqi that he asked al-Qasim ibn Muhammad about a man who made divorce conditional on his marrying a woman i.e. if he married her he would automatically divorce her. Al-Qasim ibn Muhammad said, “If a man marries a woman whom he has made as his mother’s back, i.e. has made haram for him, Umar ibn al-Khattab ordered him not to go near her if he married her until he had done the kaffara for pronouncing dhihar.”