Musnad Ahmad Hadith 27445

It is narrated from Umm Habibah (may Allah be pleased with her) that once she presented herself before the Prophet (ﷺ) and complained about continuous menstrual bleeding. The Prophet (ﷺ) 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."