Musannaf Ibn Abi Shayba 1:1652

Ibn Numayr and ‘Abdah narrated to us, from Hisham ibn ‘Urwah, from ‘Amr ibn Khuzaymah, from ‘Amarah ibn Khuzaymah, from Khuzaymah ibn Thabit, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Purification (istitabah) is with three stones, provided there is no dung (raji') in them."