‘Urwa b. az-Zubair reported from Fatima daughter of Abu Hubaish that her blood kept flowing, so the Prophet said to her, “When the blood of the menses comes it is black blood which can be recognised, so when that comes refrain from prayer; but when a different type comes perform ablution any pray, for it is due only to a vein.”
Abu Dawud and Nasa’i transmitted it.