From Sahih Bukhari (Hadith 5252):
Narrated Anas bin Seereen:Ibn `Umar said: "I divorced my wife while she was menstruating. `Umar mentioned that to the Prophet . The Prophet (ﷺ) 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 (ﷺ) 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?
Reference: Bukhari 5252