Musannaf Ibn Abi Shayba 19:746

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Hajjaj, from al-Hasan ibn Sa'id, from 'Abd al-Rahman ibn 'Abd Allah ibn Mas'ud, from 'Ali, who said: "O people, indeed adultery is of two types: secret adultery and public adultery. As for secret adultery, it is when witnesses testify, so the witnesses are the first to throw [stones], then the Imam, then the people. As for public adultery, it is when pregnancy becomes apparent or there is a confession, so the Imam is the first to throw." He said: And in his hand were three stones. He said: So he threw one stone at her, and it hit her ear canal, so she spun around, and the people threw.