Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with him) narrates: Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) wrote to Sayyiduna Amr bin al-As (may Allah be pleased with him): "Peace be upon you! To proceed: your letter has reached me, in which you mentioned that the Romans have gathered their armies. Allah Almighty did not help us alongside His Prophet through great numbers or through many armies. We used to go on campaigns with the Prophet (peace and blessings be upon him) with only a few horses among us, and we used to ride camels by turns. On the day of Uhud we were with the Prophet (peace and blessings be upon him), and we had only one horse, which the Prophet (peace and blessings be upon him) was riding; yet despite this Allah Almighty made us prevail over our opponents and helped us. O Amr! Know that the most obedient of people to Allah Almighty is the one who most hates sins. So obey Allah Almighty and order your companions to obey Him too."
Al-Tabarani recorded it; its chain contains al-Shadhakuni and al-Waqidi, and both of them are da'if (weak).