Musnad Abu Ya'la al-Mawsili 1:50

Abu Musa Ishaq ibn Ibrahim al-Harawi narrated to us, Sufyan narrated to us, from al-Walid ibn Kathir, from Ibn Tadras, the freed slave of Hakim ibn Hizam, from Asma bint Abi Bakr, that they said to her: "What is the most severe thing you saw the polytheists reach regarding the Messenger of Allah, may Allah bless him and grant him peace?" She said: "The polytheists were sitting in the mosque discussing the Messenger of Allah, may Allah bless him and grant him peace, and what he said about their gods. While they were like that, the Messenger of Allah, may Allah bless him and grant him peace, approached, so they all stood up to him together. A crier came to Abu Bakr and it was said: 'Reach your companion!' He went out from us, and he had four braids, saying: 'Do you kill a man because he says, "My Lord is Allah," while he has brought you clear proofs from your Lord?' (Ghafir). So they were distracted from the Messenger of Allah, may Allah bless him and grant him peace, and turned toward Abu Bakr. She said: Abu Bakr returned to us, and he would not touch any of his braids except that it came away with his hand, while he was saying: 'Blessed are You, O Possessor of Majesty and Honor.'"