I said to my wife: "If you become pure from menstruation, then you are divorced."

If a husband conditions his wife's divorce upon her purification from menstruation, then if the wife was pure at the time the divorce was conditioned, the divorce does not occur until after her menstruation and subsequent purification. However, if she was menstruating at the time the divorce was conditioned, she is divorced upon her purification from that menstruation. If the divorce occurs and it is not the third and final divorce, it is permissible to take her back before her waiting period (iddah) expires.