Musnad Ahmad

It is narrated from Hazrat Umm Habibah (may Allah be pleased with her) that once she presented herself before the Prophet (peace and blessings be upon him) and complained about continuous menstrual bleeding. The Prophet (peace and blessings be upon him) instructed her to perform ghusl at the time of every prayer. Thus, when she would come out of the tub, the water would have become predominantly red, yet she would perform the prayer. He said to her, "This is blood from a vein, so observe: when the time of your menstrual period comes, do not perform the prayer, and when that period passes, consider yourself pure, perform purification, and continue to pray until the next period."