Musannaf Ibn Abi Shayba 1:1651

Waki’ and Abu Mu’awiyah narrated to us, from al-A’mash, from Ibrahim, from ‘Abd al-Rahman ibn Yazid, from Salman, who said: We were commanded—meaning by the Prophet, peace and blessings of Allah be upon him—to perform istinja (cleaning oneself after defecation) with three stones. There should be no dung or bone among them.