Mukhtasar Bukhari

Narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that: I divorced my wife during the time of the Messenger of Allah (peace and blessings be upon him) while she was menstruating. (My father) Sayyiduna Umar bin Khattab (may Allah be pleased with him) asked the Messenger of Allah (peace and blessings be upon him) about this, so he (peace and blessings be upon him) said: "Command him to take her back, then keep her until she becomes pure, then when her period comes and she becomes pure again, then if he wishes he may keep her, and if he wishes he may divorce her before having intercourse with her. This is the waiting period regarding which Allah has commanded that women should be divorced at such a time."