Musnad Ahmad 26:632

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Isma'il ibn Raja', he said: I heard 'Abdullah ibn Abi al-Hudhayl narrating from Abu al-Ahwas, he said: I heard 'Abdullah ibn Mas'ud narrating from the Prophet, may Allah bless him and grant him peace, he said: "If I were to take a *khalil* (intimate friend), I would have taken Abu Bakr as a *khalil*. But he is my brother and my companion, and your companion has indeed been taken by Allah, the Almighty and Majestic, as a *khalil*."