Sahih Bukhari Volume 7, Book 63, Hadith Number 179.

Narrated By Anas bin Sirin : Ibn ‘Umar said: “I divorced my wife while she was menstruating. ‘Umar mentioned that to the Prophet. The Prophet said, (to my father), “Let your son take her back.” I asked (Ibn ‘Umar), “Is such a divorce counted (i.e. as one legal divorce)?” Ibn ‘Umar said, “Of course.” Narrated Yunus bin Jubair: Ibn ‘Umar said, “The Prophet said to ‘Umar, ‘Order him (Ibn ‘Umar) to take her back.’ ” I asked, “Is such a divorce counted (as one legal divorce)?” Ibn ‘Umar said, “What do you think if someone becomes helpless and foolish?”