Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 44
Narated By Abu Hurayrah : The Prophet (pbuh) said: The following verse was revealed in connection with the people of Quba': "In it are men who love to be purified" (ix.108). He (AbuHurayrah) said: They used to cleanse themselves with water after easing. So the verse was revealed in connection with them.