From Sahih Bukhari (Hadith 5333):
Narrated Yunus Ibn Jubair:Ibn `Umar divorced his wife while she was having her menses. `Umar asked the Prophet (ﷺ) who said, "Order him (your son) to take her back, and then divorced her before her period of the 'Iddah has elapsed." I asked Ibn `Umar, "Will that divorce (during the menses) be counted?" He replied, "If somebody behaves foolishly (will his foolishness be an excuse for his misbehavior)?
Reference: Bukhari 5333