Musnad Ahmad 944:23

Ibn Numayr narrated to us, from Hisham, [who said]: 'Amr ibn Khuzaymah narrated to me, from 'Umarah ibn Khuzaymah, from his father, Khuzaymah ibn Thabit: That the Messenger of Allah, peace and blessings of Allah be upon him, was asked about *al-istitabah* (cleansing oneself after relieving oneself), and he said: "Three stones, provided there is no dung among them."