Musnad Abu Ya'la al-Mawsili 152:240

And from Hashim, Shu'bah narrated to us, from Abu Ishaq, from Abu al-Ahwas, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If I were to take a close friend (khalil) from my nation, I would have taken Abu Bakr as a close friend."