Sahih Bukhari Volume 001, Book 003, Hadith Number 130.
Narated By Anas bin Malik : "Once Mu'adh was along with Allah's Apostle as a companion rider. Allah's Apostle said, "O Mu'adh bin Jabal." Mu'adh replied, "Labbaik and Sa'daik. O Allah's Apostle!" Again the Prophet said, "O Mu'adh!" Mu'adh said thrice, "Labbaik and Sa'daik, O Allah's Apostle!" Allah's Apostle said, "There is none who testifies sincerely that none has the right to be worshipped but Allah and Muhammad is his Apostle, except that Allah, will save him from the Hell-fire." Mu'adh said, "O Allah's Apostle ! Should I not inform the people about it so that they may have glad tidings?" He replied, "When the people hear about it, they will solely depend on it." Then Mu'adh narrated the above-mentioned Hadith just before his death, being afraid of committing sin (by not telling the knowledge).