Musannaf Ibn Abi Shayba 19:570

Abu Bakr narrated to us, he said: 'A'idh ibn Habib narrated to us, from Ibn Abi 'Aruba, from Qatada, from Khallas, who said: A man was brought, and with him were four [witnesses]. Three of them testified to adultery, but the fourth did not proceed [with the testimony]. 'Ali flogged the three [witnesses] and shaved the head of the one who was testified against.