"Ibn Abbas narrates that Hadhrath Umar went before Rasulullah (s) and "Master I am destroyed!'. Rasulullah (s) asked 'what thing has destroyed you?'. Umar replied last night I had anal sex. Rasulullah (s) did not give a reply to Umar, then Allah (swt) sent down this revelation "Your wives are as a tilth unto you; so approach your tilth when or how ye will; the words 'kabool wa Dhabar' (the anus is accepted)"

Jami al Tirmidhi, Bab al Tafseer Volume 2, page 382, 'Ayat Hars'