Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 286
Narated By Fatimah daughter of Abu Hubaysh : Urwah ibn az-Zubayr reported from Fatimah daughter of Abu Hubaysh that her blood kept flowing, so the Prophet (pbuh) 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 of blood comes, perform ablution and pray, for it is (due only to) a vein.