Musannaf Ibn Abi Shayba 27:174

'Affan narrated to us, he said: Hammad ibn 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. He said: And he was known, while the Prophet, may Allah bless him and grant him peace, was not known. So they would say, "O Abu Bakr, who is this young man in front of you?" He would say, "This is a guide who shows the way." He said: When they drew near to Al-Madinah, they dismounted at Al-Harrah, and we sent to 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. 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. May the blessings of Allah, His mercy, and His pleasure be upon him until the Day of Judgment.