Musnad Ahmad 32:1648

'Affan narrated to us, Shu'bah narrated to us, from Qatadah, from Anas ibn Malik, from the Prophet, peace and blessings of Allah be upon him, who said: "There are three things, whoever possesses them will find the sweetness of faith: that Allah and His Messenger are more beloved to him than anything else besides them; that he loves a servant, not loving him except for the sake of Allah; and that he would rather be cast into the fire than return to disbelief."