It is narrated from Nafi' (may Allah have mercy on him) that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both), in the blessed time of the Messenger of Allah (peace and blessings be upon him), gave his wife a single divorce while she was menstruating, so Sayyiduna Umar (may Allah be pleased with him) asked the Prophet (peace and blessings be upon him) about it.
Thereupon the Messenger of Allah (peace and blessings be upon him) ordered him that "he should take his wife back, then keep her with him until she becomes pure, then she should go through another menstruation while staying with him and then become pure; after that, if he wishes to divorce her, he should divorce her before having intercourse with her, for that is the waiting period ('iddah) which Allah, the Mighty and Majestic, has commanded."