Musnad Ahmad 5:414

Yahya ibn Sa'id narrated to us, from Mujalid, 'Amir narrated to us, he said: Sharaha had a husband who was absent in Ash-Sham, and she became pregnant. Her master brought her to 'Ali ibn Abi Talib and said: "This woman has committed adultery and has confessed." So he flogged her one hundred lashes on Thursday, and he stoned her on Friday. He dug a hole for her up to her navel, and I was a witness. Then he said: "Indeed, stoning is a Sunnah established by the Messenger of Allah, peace and blessings of Allah be upon him. If anyone had testified against her, he would have been the first to throw; the witness testifies, then follows his testimony with his stone. But she confessed, so I am the first to throw at her." He threw a stone at her, then the people threw, and I was among them. He said: "And I was, by Allah, among those who killed her."