Musnad Abu Ya'la al-Mawsili 128:7

Muhammad ibn Abdullah ibn Numayr narrated to us, Ibn Abi Ubaydah narrated to us, my father narrated to me, from Abu Sufyan, from Anas ibn Malik, who said: They struck the Messenger of Allah, may Allah bless him and grant him peace, one time until he fainted. Then Abu Bakr, may Allah be pleased with him, stood up and hurried, calling out: "Woe to you! Do you kill a man for saying, 'My Lord is Allah'?" They said: "Who is this?" He said: "The son of Abu Quhafah, the madman."