Musnad Ahmad 944:30

Yahya ibn Sa'id narrated to us, Hisham narrated to us, from his father, from the Prophet, peace and blessings of Allah be upon him, who said regarding *istinja* (cleaning oneself after relieving oneself): "Does one of you not find three stones?" And a man informed me, from 'Umarah ibn Khuzaymah ibn Thabit, from his father, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Three stones, in which there is no dung."