Musannaf Ibn Abi Shayba 9:769

Muhammad ibn al-Hasan narrated to us, he said: Sharik narrated to us, from ‘Ata’ ibn al-Sa’ib, from Sa’id ibn Jubayr, regarding the saying of the Exalted: "Your wives are a tilth for you, so come to your tilth when or how you will." He said: He may come to her from her front and from her back, as long as it is not in the anus.