Sahih Bukhari Volume 001, Book 005, Hadith Number 291.
Narated By Zaid bin Khalid AjJuhani : I asked 'Uthman bin 'Affan about a man who engaged in the sexual intercourse with his wife but did not discharge. 'Uthman replied, "He should perform ablution like that for the prayer after washing his private parts." 'Uthman added, "I heard that from Allah's Apostle." I asked 'Ali bin Abi Talib, Az-Zubair bin Al-'Awwam, Talha bin 'Ubaidullah and Ubai bin Ka'b and a gave the same reply. (Abu Aiylub said that he had heard that from Allah's Apostle) (This order was cancelled later on so one has to take a bath. See, Hadith No. 180).