Musnad Ahmad 541:5

Muhammad ibn Ja'far and Hajjaj narrated to us, they said: Shu'bah narrated to us, from 'Amr ibn Dinar, from a man from the people of Egypt who narrated from 'Amr ibn al-'As, that he said: Muhammad ibn Abi Bakr was taken captive. He said: So 'Amr began to ask him, desiring that he would claim a guarantee of safety. He said: Then 'Amr said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The lowest of them [the Muslims] may grant protection on behalf of the Muslims."