Musnad Ahmad 25:574

Ishaq ibn Isa narrated to us, Jarir narrated to us, from Ya'la ibn Hakim, from Ikrimah, from Ibn Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) went out during his illness in which he died, with his head bound in a cloth. He sat upon the pulpit, praised Allah and extolled Him, then said: "There is no one who has been more generous to me with his self and his wealth than Abu Bakr ibn Abi Quhafa. If I were to take a close friend (khalil) from among the people, I would have taken Abu Bakr as a close friend, but the brotherhood of Islam is better. Close off every small door (khawkha) leading into this mosque for me, except for the door of Abu Bakr."