Musnad Abu Ya'la al-Mawsili 152:181

Narrated to us Abd al-Rahman, from Shu'bah, from Isma'il ibn Raja', from Abd Allah ibn Abi al-Hudhayl, from Abu al-Ahwas, from Abd Allah, from the Prophet, may Allah bless him and grant him peace, who 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 Allah has taken your companion as a *khalil*."