Musnad Ahmad 1156:171

'Abd al-Razzaq narrated to us, he said: Ma'mar narrated to us, from Ibn Khuthaym, from Ibn Sabit, from Hafsah bint 'Abd al-Rahman, from Umm Salamah. [Abdullah said:] My father said: And in another place: Ma'mar, from Ibn Khuthaym, from Safiyyah bint Shaybah, from Umm Salamah, that a woman asked her about a man who approaches his wife from behind (mujbiyah). Umm Salamah asked the Messenger of Allah, peace and blessings of Allah be upon him, and he said: "Your wives are a tilth for you, so come to your tilth when or how you will" [Al-Baqarah: 223], in one passage (simaman wahidan).