Sunan An-Nasai, Chapter: The Book of Purification

that a woman suffered constant bleeding at the time of the Messenger of Allah (saw), so Umm Salamah consulted the Prophet (saw) for her. He said: "Let her count the number of nights and days that she used to menstruate each month before this happened to her, and let her stop praying for that amount of time each month. Then when that is over let her perform Ghusl, then let her use a pad, and pray." (Daif)