Majma al-Zawaid 15 — One who testifies that there is no god but Allah

Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) reported about the Prophet (peace and blessings be upon him) that one day he (peace and blessings be upon him) said: "Whoever says La ilaha illallah (there is no god but Allah), Paradise will become obligatory for him." Sayyiduna Mu'adh (may Allah be pleased with him) asked the Prophet (peace and blessings be upon him) for permission to take this to the people and give them the glad tidings, and the Prophet (peace and blessings be upon him) gave him permission. Sayyiduna Mu'adh (may Allah be pleased with him) set out quickly in a state of joy. On the way he met Sayyiduna Umar (may Allah be pleased with him), and Sayyiduna Umar (may Allah be pleased with him) asked: "What is the matter with you?" Sayyiduna Mu'adh (may Allah be pleased with him) told him about it, and Sayyiduna Umar (may Allah be pleased with him) said: "Stay where you are and do not hurry!" Then Sayyiduna Umar (may Allah be pleased with him) came to the Prophet (peace and blessings be upon him) and said: "O Prophet of Allah! Your opinion (peace and blessings be upon him) is certainly the best, but if the people hear about this, they will rely on it alone and will not act." The Prophet (peace and blessings be upon him) said: "Call him back! Call him back!"
Imam al-Bazzar recorded it; its chain contains a narrator, Muhammad bin Abi Layla, who has been declared da'if (weak).