Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: Sayyiduna Umar (may Allah be pleased with him) came to the Prophet (peace and blessings be upon him) and said: O Messenger of Allah, I am ruined! He (peace and blessings be upon him) asked: "What has ruined you?" He said: Last night I turned my saddle around (i.e. approached my wife from behind). He gave him no reply, and then revelation came to him (peace and blessings be upon him): "Your wives are a tilth for you" [Surah al-Baqarah: 223]. "Come from the front or from behind, but avoid the backside and menstruation."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14125