Musnad Abu Ya'la al-Mawsili 149:20

Abu Ma'mar narrated to us, 'Abd al-'Aziz ibn Abi Hazim narrated to us, from his father, from Muslim ibn Qurt, from 'Urwah, from 'Aishah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you goes to the privy, let him cleanse himself with three stones, for they are sufficient for him."