Musnad Abu Ya'la al-Mawsili 111:124

Zuhayr narrated to us, Yunus ibn Muhammad narrated to us, Ya'qub al-Qummi narrated to us, Ja'far narrated to us, from Sa'id ibn Jubayr, from Ibn Abbas, who said: Umar ibn al-Khattab came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "I am ruined." He said: "And what has ruined you?" He said: "I turned my saddle (i.e., changed my position of intercourse) last night." He said: "He did not reply to him with anything." He said: "Then it was revealed to the Messenger of Allah, may Allah bless him and grant him peace, this verse: 'Your wives are a tilth for you, so come to your tilth when or how you will' (Al-Baqarah)." He said: "Meaning: come from the front and come from the back, but avoid the anus and the time of menstruation."