Musnad Ahmad 26:142

Waki’ narrated to us, al-A’mash narrated to us, from ‘Abdullah ibn Murrah, from Abu al-Ahwas, from ‘Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Behold, I disassociate myself from every intimate friend (khalil) because of his intimate friendship. And if I were to take an intimate friend, I would have taken Abu Bakr as an intimate friend. Indeed, your companion is the intimate friend of Allah, the Almighty and Majestic."