Musannaf Ibn Abi Shaybah

Hazrat Abdur Rahman bin Abi Laila narrates that when witnesses would testify to adultery before Hazrat Ali (may Allah be pleased with him), he would order the witnesses to throw stones first, then he would throw stones, then the people would throw stones. And when there was a confession, he himself would begin, so he would throw stones, then the people would throw stones.