It is narrated from Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "If I were to take anyone besides my Lord as an intimate friend, I would have taken Abu Bakr as an intimate friend; but he is my brother and my companion in the cave."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13883