Yahya related to me from Malik from Nafi that Abdullah ibn Umarsaid, "When a man makes a vow to abstain from intercourse with hiswife and four months have passed he must declare his intent and eitherhe is divorced or he revokes his vow. Divorce does not occur untilfour months have passed and he continues to abstain."
USC-MSA web (English) reference: Book 29, Hadith 18
Arabic reference: Book 29, Hadith 1171