Sahih Bukhari Volume 007, Book 062, Hadith Number 061.
Narrated By Al-Hasan : Concerning the Verse: 'Do not prevent them' (2.232) Ma'qil bin Yasar told me that it was revealed in his connection. He said, "I married my sister to a man and he divorced her, and when her days of 'Idda (three menstrual periods) were over, the man came again and asked for her hand, but I said to him, 'I married her to you and made her your bed (your wife) and favored you with her, but you divorced her. Now you come to ask for her hand again? No, by Allah, she will never go back to you (again)!' That man was not a bad man and his wife wanted to go back to him. So Allah revealed this Verse: 'Do not prevent them.' (2.232) So I said, 'Now I will do it (let her go back to him), O Allah's Apostle."So he married her to him again.