Musnad Abu Ya'la al-Mawsili 209:94

Abu Khaythama narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Ibn Khuthaym, from Ibn Sabit, from Hafsa bint Abd al-Rahman, from Umm Salama, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Your wives are a tilth for you, so come to your tilth however you wish." He said: "One opening, one opening."