It is narrated from Hazrat Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said to Hazrat Mu'adh (may Allah be pleased with him): Whoever meets Allah in such a state that he does not associate anyone with Him, he will enter Paradise. He (Mu'adh) said, "O Messenger of Allah! Should I not give this glad tidings to the people?" The Prophet (peace and blessings be upon him) said, "No, I fear that they will rely on it and become complacent."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 12606