Musannaf Ibn Abi Shayba 35:261

'Abdah ibn Sulayman narrated to us, from Hisham ibn 'Urwah, from Abu Khuzaymah, from 'Umarah ibn Khuzaymah, from Khuzaymah ibn Thabit, who said: The Prophet (peace be upon him) said regarding purification (al-istitabah): "Three stones, provided there is no dung among them."