Musannaf Ibn Abi Shayba 19:752

Abu Bakr narrated to us, he said: Abu Usama narrated to us, from Awf, from Qasama ibn Zuhayr, who said: When the affair of Abu Bakra and al-Mughira ibn Shu'ba occurred, Abu Bakra said: "Avoid or move away from our prayer, for we do not pray behind you." He said: So he wrote to Umar regarding his affair. He said: So Umar wrote to al-Mughira: "To proceed: A report about you has reached me, and if it is true regarding you, then for you to have died before today would have been better for you." He said: He wrote to him and to the witnesses to come to him. When they arrived, he summoned the witnesses, and they testified. Abu Bakra, Shibl ibn Ma'bad, and Abu Abd Allah Nafi' testified. When these three testified, Umar said: "I wish al-Mughira had four [witnesses]," and the affair weighed very heavily upon Umar. When Ziyad stood up, he said: "I will not testify, if Allah wills, except to the truth." Then he testified and said: "As for adultery, I do not testify to it, but I saw an ugly matter." Umar said: "Allah is the Greatest! Flog them." So they flogged them. When he finished flogging Abu Bakra, Abu Bakra stood up and said: "I testify that he is an adulterer." Umar intended to repeat the punishment upon him, but Ali said: "If you flog him, then stone your companion [al-Mughira]." So he left him and did not flog him, and he did not accuse [anyone] of adultery twice after that.