Sahih Muslim 148

It is reported on the authority of Anas b. Malik 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