Musannaf Ibn Abi Shayba 19:745

Abu Bakr narrated to us, he said: Abdullah ibn Idris narrated to us, from Yazid, from Abd al-Rahman ibn Abi Layla, that Ali, when witnesses testified before him regarding adultery, would command the witnesses to stone [the accused], then he would stone [the accused] himself, and then the people would stone [the accused]. And when it was a confession, he would begin and stone [the accused] himself, then the people would stone [the accused].