Musnad Ahmad 1156:39

Ibn Numayr narrated to us, Ubayd Allah narrated to us, from Nafi’, from Sulayman ibn Yasar, from Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, that she asked the Messenger of Allah, may Allah bless him and grant him peace, for a legal opinion regarding a woman who experiences a flow of blood. He said: "She shall wait for the duration of the nights and days that she used to menstruate, and their duration within the month, then she shall leave the prayer. Then, she shall perform ghusl, bind herself with a cloth, and then pray."