Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said concerning the statement of Allah "Your wives are a tilth for you, so come to your tilth however you wish" [Surah al-Baqarah: 223]: You may come from the front or from behind, but in the private part.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14109