Abu Dawud Book 1, Hadith Number 214.

Chapter : Bathing is obligatory after sexual intercourse without seminal emmision.

Narated By Ubayy b. Ka’b : The Apostle of Allah (may peace be upon him) made a concession in the early days of Islam on account of the paucity of clothes that one should not take a bath if one has sexual intercourse (and has no seminal emission). But later on he commanded to take a bath in such a case and prohibited its omission.