Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Allah, the Mighty and Majestic, looked into the hearts of the servants and found the heart of Muhammad (peace and blessings be upon him) to be the best of the hearts of the servants, so He chose him for Himself and singled him out - or he said: sent him with His message. Then, after his (peace and blessings be upon him) heart, He looked into the hearts of the servants and found the hearts of his (peace and blessings be upon him) Companions to be the best of the hearts of the servants, so He made them the ministers (helpers) of His Prophet (peace and blessings be upon him), fighting for the sake of his (peace and blessings be upon him) religion."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8550