Sahih Muslim 1471 a
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 'Umar (Allah be pleased with them) reported that he divorced his wife while she was menstruating during the lifetime of Allah's Messenger (ﷺ). 'Umar b. Khattib (Allah be pleased with him) asked Allah's Messenger (ﷺ) about it, whereupon Allah's Messenger (ﷺ) said: Command him ('Abdullah b. 'Umar) to take her back (and keep her) and pronounce divorce when she is purified and she again enters the period of menstruation and she is again purified (after passing the period of menses), and then if he so desires he may keep her and if he desires divorce her (finally) before touching her (without having an intercourse with her), for that is the period of waiting ('ldda) which God, the Exalted and Glorious, has commanded for the divorce of women.