Sahih Bukhari Volume 001, Book 004, Hadith Number 228.
Narated By 'Aisha : Fatima bint Abi Hubaish came to the Prophet and said, "O Allah's Apostle I get persistent bleeding from the uterus and do not become clean. Shall I give up my prayers?" Allah's Apostle replied, "No, because it is from a blood vessel and not the menses. So when your real menses begins give up your prayers and when it has finished wash off the blood (take a bath) and offer your prayers." Hisham (the sub narrator) narrated that his father had also said, (the Prophet told her): "Perform ablution for every prayer till the time of the next period comes."