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

In the time of the Apostle of Allah (peace_be_upon_him) there was a woman who had an issue of blood. So Umm Salamah asked the Apostle of Allah (peace_be_upon_him) to give a decision about her. He said: She should consider the number of nights and days during which she used to menstruate each month before she was afflicted with this trouble and abandon prayer during that period each month. When those days and nights are over, she should take a bath, tie a cloth over her private parts and pray.