Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 215
Narated By Ubayy b. Ka'b : The verdict that water (bath) is necessary when there is emission given by the people (in the early days of Islam) was due to the concession granted by the Apostle of Allah in the beginning of Islam. He then commanded to take a bath (in such a case).
Abu Dawud said: By Abu Ghassän is meant Muhammad b. Mutarrif.