Musnad Ahmad 1153:994

Sa'id ibn Mansur narrated to us, he said: Ya'qub ibn 'Abd al-Rahman narrated to us, from Abu Hazim, from Muslim ibn Qurt, from 'Urwah ibn al-Zubayr, from 'Aishah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you goes to relieve himself, let him take with him three stones to cleanse himself with, for they are sufficient for him."