Does a divorce occur with the words "you are divorced" uttered once, if the wife was menstruating when the husband pronounced it, without him knowing of her menstruation, and what must the husband do in this case?

Separation occurs with an explicit divorce. If the wife is divorced in an innovated (bid‘i) manner, such as while she is menstruating, then according to the Hanafis and Malikis, revoking the divorce (raj‘a) is obligatory. According to the Shafi‘is and Hanbalis, it is sunnah. This is done by saying: "I take my wife back" or by engaging in intercourse. The evidence for this is the hadith of Ibn Umar concerning his divorce of his wife while she was menstruating.