Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) narrates that 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 mention that the Romans have gathered great armies. Allah Almighty did not help us, when we were with His Prophet (peace and blessings be upon him), through great numbers nor through many troops. We used to go on expeditions with the Messenger of Allah (peace and blessings be upon him) with nothing but a few horses, and we used to ride the camels in turns. On the day of Uhud we were with the Messenger of Allah (peace and blessings be upon him) and we had only one horse, which the Messenger of Allah (peace and blessings be upon him) was riding; and (despite that) Allah used to make us prevail and help us against those who opposed us. And know, O Amr, that the most obedient of people to Allah is the one who hates sins the most; so obey Allah and command your companions to obey Him."
Imam al-Tabarani (may Allah have mercy on him) said: "This hadith is narrated from Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) with a connected chain only through this chain; al-Waqidi is alone in narrating it."