Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 247
Narated By Abdullah ibn Umar : There were fifty prayers (obligatory in the beginning); and (in the beginning of Islam) washing seven times because of sexual defilement (was obligatory); and washing the urine from the cloth seven times (was obligatory).
The Apostle of Allah (pbuh) kept on praying to Allah until the number of prayers was reduced to five and washing because of sexual defilement was allowed only once and washing the urine from the clothe was also permitted only once.