Musnad Ahmad 26:862

'Affan narrated to us, Shu'bah narrated to us, from Isma'il ibn Raja', from 'Abdullah ibn Abi al-Hudhayl, from Abu al-Ahwas, from 'Abdullah, from the Prophet, peace and blessings of Allah be upon him, who said: "If I were to take a *khalil* (intimate friend), I would have taken Abu Bakr as a *khalil*. However, he is my brother and my companion, and your companion has already been taken by Allah as a *khalil*."