Sahih Bukhari Volume 7, Book 63, Hadith Number 178.

Narrated By ‘Abdullah bin ‘Umar : That he had divorced his wife while she was menstruating during the lifetime of Allah’s Apostle. ‘Umar bin Al-Khattab asked Allah’s Apostle about that. Allah’s Apostle said, “Order him (your son) to take her back and keep her till she is clean and then to wait till she gets her next period and becomes clean again, whereupon, if he wishes to keep her, he can do so, and if he wishes to divorce her he can divorce her before having sexual intercourse with her; and that is the prescribed period which Allah has fixed for the women meant to be divorced.”