Musannaf Ibn Abi Shayba 10:607

Abu Bakr narrated to us, he said: Sahl ibn Yusuf narrated to us, from Shu'bah, from al-Hakam and Hammad, they both said: If a man says to his wife, "You are to me like the back of my mother for four months," and four months pass, then it is *Ila'* (oath of continence). And if he says, "You are to me like the back of my mother," and he leaves her for a year, then it is not *Ila'*.