Sahih Bukhari Volume 7, Book 63, Hadith Number 213.

Narrated By Nafi : Ibn ‘Umar used to say about the Ila (which Allah defined (in the Holy Book), “If the period of Ila expires, then the husband has either to retain his wife in a handsome manner or to divorce her as Allah has ordered.” Ibn ‘Umar added, “When the period of four months has expired, the husband should be put in prison so that he should divorce his wife, but the divorce does not occur unless the husband himself declares it. This has been mentioned by ‘Uthman, ‘Ali, Abu Ad-Darda, ‘Aisha and twelve other companions of the Prophet.”