Maliks Muwatta Book 29, Hadith Number 17.

Section : Annulment of Marriage by Husband’s Vow to Refrain from Intercourse (Ila).

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.”