Sahih Bukhari Volume 001, Book 006, Hadith Number 324.
Narated By 'Aisha : (The wife of the Prophet) Um Habiba got bleeding in between the periods for seven years. She asked Allah's Apostle about it. He ordered her to take a bath (after the termination of actual periods) and added that it was (from) a blood vessel. So she used to take a bath for every prayer.