Sahih Muslim 1471 j
The Book of Divorce
The prohibition of divorcing a menstruating woman without her consent; If a man breaks this rule, it still counts as a divorce, and he should be ordered to take her back
Ibn Sirin reported: One who was blameless (as a narrator) narrated to me for twenty years that Ibn 'Umar (Allah be pleased with him) pronounced three divorces to his wife while she was in the state of menses. He was commanded to take her back. I neither blamed them (the narrators) nor recognised the hadith (to be perfectly genuine) until I met Abu Ghallab Yunus b. Jubair al-Bahili and he was very authentic, and he narrated to me that he had asked Ibn 'Umar (Allah be pleased with there) and he narrated it to him that he made one pronouncement of divorce to his wife as she was in the state of menses, but he was commanded to take her back. I said: Was it counted (as one pronouncement)? He said: Why not, was I helpless or foolish?