Maliks Muwatta Book 2, Hadith Number 107.

Yahya related to me from Malik from Nafi from Sulayman ibn Yasar from Umm Salama, the wife of the Prophet, may Allah bless him and grant him peace, that a certain woman in the time of the Messenger of Allah, may Allah bless him and grant him peace, used to bleed profusely, so Umm Salama consulted the Messenger of Allah, may Allah bless him and grant him peace, for her, and he said, “She should calculate the number of nights and days a month that she used to menstruate before it started happening, and she should leave off from prayer for that much of the month. When she has completed that she should do ghusl, bind her private parts with a cloth, and then pray.”