Musannaf Ibn Abi Shayba 19:748

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from al-Hakam, he said: I heard 'Amr ibn Nafi' narrating from 'Ali, he said: "Stoning is of two types: the Imam stones, then the people; and a stoning where the witnesses stone, then the Imam, then the people." I said to al-Hakam: "What is the stoning of the Imam?" He said: "When she gives birth or confesses." "And the stoning of the witnesses?" He said: "When they testify."