Musnad Ahmad 25:556

Yahya ibn Ghaylan narrated to us, Rashdin narrated to us, Hassan ibn Thawban narrated to me, from Amir ibn Yahya al-Mu'afiri, Hanash narrated to me, from Ibn Abbas, he said: This verse was revealed, "Your wives are a tilth for you" [Al-Baqarah: 223], regarding some people from the Ansar who came to the Prophet, may Allah bless him and grant him peace, and asked him. The Messenger of Allah, may Allah bless him and grant him peace, said: "Approach her in any manner, as long as it is in the vagina."