Imam Malik's Muwatta, Chapter: Divorce

Yahya related to me from Malik from Jafar ibn Muhammad from his father that Ali ibn Abi Talib said, "When a man takes a vow to abstain from intercourse, divorce does not occur immediately. If four months pass, he must declare his intent and either he is divorced or he revokes his vow. " Malik said, "That is what is done among us."