Sunan Abu Dawood, Chapter: Purification (Kitab Al-Taharah)

Abu Salamah said: Zaynab daughter of AbuSalamah reported to me that a woman had a copious flow of blood. She was the wife of AbdurRahman ibn Awf. The Apostle of Allah (peace_be_upon_him) commanded her to take a bath at the time of every prayer, and then to pray. He reported to me that Umm Bakr told him that Aisha said: The Apostle of Allah (peace_be_upon_him) said about a woman who was doubtful of her menstruation after purification that it was a vein or veins.