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 madedivorce conditional on his marrying a woman i.e. if he married her hewould automatically divorce her. Al-Qasim ibn Muhammad said, "If a manmarries a woman whom he has made as his mother's back, i.e. has madeharam for him, Umar ibn al-Khattab ordered him not to go near her ifhe married her until he had done the kaffara for pronouncing dhihar."
USC-MSA web (English) reference: Book 29, Hadith 20
Arabic reference: Book 29, Hadith 1176