Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with this same chain (it is narrated) from (Sayyida Aisha, may Allah be pleased with her) that Fatima bint Abi Hubaysh, may Allah be pleased with her, said to the Messenger of Allah, peace and blessings be upon him: "I do not become pure, so should I leave the prayer?" The Messenger of Allah, peace and blessings be upon him, said: "This is a vein (blood), it is not menstruation. When the time of menstruation comes, then leave the prayer, then when the days (of menstruation) have passed according to its calculation, wash the blood from yourself and perform the prayer."