Musnad Abu Ya'la al-Mawsili 209:16

Abdullah ibn Muhammad ibn Asma’ narrated to us, Juwayriyah narrated to us, from Nafi’, that Sulayman ibn Yasar informed him that a man informed him from Umm Salamah, the wife of the Prophet (may Allah bless him and grant him peace): That a woman was experiencing blood flow during the time of the Messenger of Allah (may Allah bless him and grant him peace), so Umm Salamah asked the Prophet (may Allah bless him and grant him peace) for a legal ruling on her behalf. The Messenger of Allah (may Allah bless him and grant him peace) said: "Let her look at the number of days she used to menstruate before what has happened to her happened, and estimate them in the months, then let her leave the prayer for that duration. Then, when that has passed and the time for prayer arrives, let her perform ghusl, bind herself with a cloth, and pray."