Musnad Ahmad 32:2119

'Affan narrated to us, Hammad narrated to us, from Thabit, from Anas, that Abu Bakr was the rider behind the Messenger of Allah (peace and blessings of Allah be upon him) between Makkah and Madinah. Abu Bakr used to travel to Ash-Sham, and he was known, while the Prophet (peace and blessings of Allah be upon him) was not known. So they would say: "O Abu Bakr, who is this young man in front of you?" He would say: "This one guides me to the path." When they drew near to Madinah, they dismounted at Al-Harrah and sent for the Ansar. They came and said: "Arise, you are safe and obeyed." He said: "I witnessed him the day he entered Madinah, and I have never seen a day that was better or more radiant than the day he entered upon us. And I witnessed him the day he died, and I have never seen a day that was uglier or darker than the day he died (peace and blessings of Allah be upon him)."