Musnad Ahmad 31:146

Abu ‘Amir narrated to us, Fulayh narrated to us, from Salim Abu al-Nadr, from Busr ibn Sa‘id, from Abu Sa‘id, who said: The Messenger of Allah (may Allah bless him and grant him peace) addressed the people and said: "Indeed, Allah the Almighty gave a servant a choice between the world and what is with Him." He said: "So that servant chose what is with Allah." He said: Abu Bakr (may Allah be pleased with him) wept. We were amazed at his weeping, for the Messenger of Allah (may Allah bless him and grant him peace) was informing us about a servant who was given a choice, and the Messenger of Allah (may Allah bless him and grant him peace) was the one given the choice, and Abu Bakr was the most knowledgeable of us regarding him. The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, the most generous of people to me with his companionship and his wealth is Abu Bakr. If I were to take a close friend (khalil) from the people other than my Lord, I would have taken Abu Bakr, but it is the brotherhood of Islam—or its affection. Let no door in the mosque remain except that it is closed, except for the door of Abu Bakr."