Musnad Ahmad 26:803

Abu Sa'id and Ibn Ja'far narrated to us, they said: Shu'bah narrated to us, Abu Ishaq narrated to us—Muhammad, meaning Ibn Ja'far, said: 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."