Musannaf Ibn Abi Shayba 36:91

'Affan narrated to us, he said: Hammad bin Salamah narrated to us, he said: Thabit informed us, from Anas, that Abu Bakr was the rider behind the Prophet, may Allah bless him and grant him peace, from Makkah to Al-Madinah. Abu Bakr used to travel to Ash-Sham, so he was known, while the Prophet, may Allah bless him and grant him peace, was not known. They would say, "O Abu Bakr, who is this young man in front of you?" He would say, "A guide who guides me to the path." He said: When they drew near to Al-Madinah, they dismounted at Al-Harrah and sent for the Ansar, so they came. He said: I witnessed him the day he entered Al-Madinah, and I have never seen a day that was more beautiful or more radiant than the day he entered upon us in it. And I witnessed the day he died, and I have never seen a day that was uglier or darker than the day he died, may Allah bless him and grant him peace.