Musnad Ahmad 28:430

Ali ibn Abdullah narrated to us, Al-Walid ibn Muslim narrated to us, Al-Awza'i narrated to me, Yahya ibn Abi Kathir narrated to me, Muhammad ibn Ibrahim ibn al-Harith al-Taymi narrated to me, Urwah ibn al-Zubayr narrated to me, he said: I said to Abdullah ibn Amr ibn al-As: Inform me of the most severe thing the polytheists did to the Messenger of Allah, may Allah bless him and grant him peace? He said: While the Messenger of Allah, may Allah bless him and grant him peace, was praying in the courtyard of the Kaaba, Uqbah ibn Abi Mu'ayt approached, took hold of the Prophet's, may Allah bless him and grant him peace, shoulder, twisted his garment around his neck, and strangled him with it severely. Then Abu Bakr, may Allah be pleased with him, approached, took hold of his [Uqbah's] shoulder, pushed him away from the Messenger of Allah, may Allah bless him and grant him peace, and said: "Do you kill a man because he says, 'My Lord is Allah,' while he has brought you clear proofs from your Lord?" [Ghafir: 28]