Sahih Bukhari Volume 001, Book 006, Hadith Number 303.
Narated By 'Aisha : Fatima bint Abi Hubaish said to Allah's Apostle, "O Allah's Apostle! I do not become clean (from bleeding). Shall I give up my prayers?" Allah's Apostle replied: "No, because it is from a blood vessel and not the menses. So when the real menses begins give up your prayers and when it (the period) has finished wash the blood off your body (take a bath) and offer your prayers."