Sahih Bukhari Volume 1, Book 6, Hadith Number 317

Narated By ‘Aisha : Fatima bint Abi Hubaish used to have bleeding in between the periods, so she asked the Prophet about it. He replied, “The bleeding is from a blood vessel and not the menses. So give up the prayers when the (real) menses begin and when it has finished, take a bath and start praying.”