Musannaf Ibn Abi Shaybah

Hazrat Ikrimah, in the explanation of the verse of the Holy Qur'an { نِسَاؤُکُمْ حَرْثٌ لَکُمْ فَأْتُوا حَرْثَکُمْ أَنَّی شِئْتُمْ } "Your wives are your fields; so come to your fields as you wish," says that it means from the front (vaginal intercourse).