Sahih al-Bukhari Book 68, Hadith 2
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?"
that he had divorced his wife while she was menstruating during the lifetime of Allah's Messenger (ﷺ) . `Umar bin Al-Khattab asked Allah's Messenger (ﷺ) about that. Allah's Messenger (ﷺ) said, "Order him (your son) to take her back and keep her till she is clean and then to wait till she gets her next period and becomes clean again, whereupon, if he wishes to keep her, he can do so, and if he wishes to divorce her he can divorce her before having sexual intercourse with her; and that is the prescribed period which Allah has fixed for the women meant to be divorced."
(Divorcing my wife during her menses) was counted as one legal divorce.