Musannaf Ibn Abi Shayba 27:288

Yunus ibn Muhammad narrated to us, he said: Fulayh ibn Sulayman narrated to us, from Salim Abu al-Nadr, from Ubayd ibn Hunayn and Busr ibn Sa’id, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) addressed the people and said: "Indeed, the most generous of the people to me with his companionship and his wealth is Abu Bakr. And if I were to take a close friend (khalil) from the people, I would have taken Abu Bakr, but [there is] the brotherhood of Islam and its affection. Let no door in the mosque remain except that it is closed, except for the door of Abu Bakr."