Musnad Ahmad 1153:754

Surayj narrated to us, Ibn Abi Hazim narrated to us, from his father, from Muslim ibn Qurt, from Urwah ibn al-Zubayr, who said: I heard Aisha saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you goes for his need, let him cleanse himself with three stones, for they are sufficient for him."