Mu'adh bin Jabal was riding on the beast with the Prophet (ﷺ), when he (ﷺ) said to him, "O Mu'adh!" Mu'adh replied, "Here I am responding to you, and at your pleasure, O Messenger of Allah." He (ﷺ) again called out, "O Mu'adh." He (again) replied, "Here I am responding your call, and at your pleasure." He (Messenger of Allah) addressed him (again), "O Mu'adh!" He replied, "Here I am responding to you, and at your pleasure, O Messenger of Allah." Upon this he (the Prophet (ﷺ)) said, "If anyone testifies sincerely that there is no true god except Allah, and Muhammad is His slave and Messenger, truly from his heart, Allah will safeguard him from Hell." He (Mu'adh) said, "O Messenger of Allah, shall I not then inform people of it, so that they may have glad tidings." He (ﷺ) replied, "Then they will rely on it alone (and thus give up good works altogether)." Mu'adh (May Allah be pleased with him) disclosed this Hadith at the time of his death, to avoid sinning for concealing.

[Al-Bukhari and Muslim].