Sahih Muslim Book 9, Hadith Number 3495.

Chapter: Atonement is essential for one who made his wife unlawful for himself without the intention of divorce.

Ibn Abbas (Allah be pleased with them) reported: When a man declares his wife unlawful for himself that is an oath which must be atoned, and he said: There is in the Messenger of Allah (may peace be upon him) a noble pattern for you.