Musnad Ahmad 32:663

'Arim narrated to us, Mu'tamir ibn Sulayman narrated to us, he said: I heard my father say: Anas ibn Malik narrated to us, that it was mentioned to him that the Prophet, peace and blessings of Allah be upon him, said to Mu'adh: "Whoever meets Allah not associating anything with Him, will enter Paradise." He said: "O Prophet of Allah, shall I not give the people the glad tidings?" He said: "No, for I fear that they will rely upon it," or as he said.