There is no one who bears witness that there is no god but Allah and that Muhammad is His slave and Messenger, sincerely from his heart, except that Allah will make him forbidden to Hellfire

Mu‘ādh ibn Jabal (may Allah be pleased with him) was riding on a mount behind the Prophet (may Allah's peace and blessings be upon him), who called him: O Mu‘ādh! He repeated the call three times to show the significance of what he was about to say. Meanwhile, Mu‘ādh was replying: "Here I am at your service, O Messenger of Allah." That's I am answering your call, O Messenger of Allah, and seek happiness in answering you.

The Prophet (may Allah's peace and blessings be upon him) told him that if anyone bears witness that there is no god but Allah - which means that none is truly worthy of worship except Allah - and that Muhammad is the Messenger of Allah, sincerely from his heart, not lying, and then he dies in this state, Allah will make him forbidden to Hellfire.

Mu‘ādh (may Allah be pleased with him) asked the Prophet (may Allah's peace and blessings be upon him) whether he could tell people about that so that they would rejoice and feel hopeful. But the Prophet (may Allah's peace and blessings be upon him) feared that they might solely rely upon that and their good deeds might decrease. So, Mu‘ādh did not narrate this Hadīth to anyone, except at the time of his death, fearing that he would incur the sin of concealing knowledge.