Abu Dawud Book 1, Hadith Number 44.

Chapter : Cleansing with water after releiving oneself.

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.