Mishkat al-Masabih

Anas reported that, when Mu'adh was riding behind the Prophet on the saddle, he said, “Mu'adh”, to which he replied, “At your service and at your pleasure, messenger of God.” He said, “Mu'adh”, to which he replied, “At your service and at your pleasure, messenger of God.” He said, “Mu'adh”, to which he replied, “At your service and at your pleasure, messenger of God,” three times altogether. He said, “If anyone testifies sincerely from his heart that there is no god but God and that Muhammad is God’s messenger, God will declare him immune from hell.” He asked, “Messenger of God, shall I not then inform people of it, so that they may be of good cheer?” He replied, “Then they would trust in it alone.” Mu'adh told about it at the time of his death, to avoid sinning. (Bukhari and Muslim.)