Sahih Muslim, Chapter: Faith (Kitab Al Iman)

that the Prophet of Allah (may peace and blessings be upon him) addressed Mu'adh b. Jabal as he was riding behind him to which he replied: At thy beck and call, and at thy pleasure, Messenger of Allah. He again called out: Mu'adh, to which he (again) replied: At thy beck and call, and at thy pleasure. He (the Holy Prophet) addressed him (again): Mu'adh, to which he replied: At thy beck and call, and at thy pleasure, Messenger of Allah. Upon this he (the Holy Prophet) observed: If anyone testifies (sincerely from his heart) that there is no god but Allah, and that Muhammad is His bondsman and His messenger, Allah immuned him from Hell. He (Mu'adh) said: Messenger of Allah, should 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.