Sahih Bukhari
Menstrual Periods
Volume: 001 / Book Number: 006 / Hadith Number: 322
Narrated 'Aisha:
Fatima bint Abi Hubaish asked the Prophet, "I got persistent bleeding (in between the periods) and do not become clean. Shall I give up prayers?" He replied, "No, this is from a blood vessel. Give up the prayers only for the days on which you usually get the menses and then take a bath and offer your prayers."