Sayyiduna Anas bin Malik (may Allah be pleased with him) said: Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) came to me, and I asked: "O Mu'adh, where are you coming from?" He said: "I am coming from the Prophet of Allah (peace and blessings be upon him)." I asked: "What did he say to you?" He said: "He (peace and blessings be upon him) said: 'Whoever bears witness with sincerity that there is no true god except Allah will enter Paradise.'" I said: "Shall I then go and ask the Prophet (peace and blessings be upon him) (myself)?" He said: "Go." So I came to the Prophet (peace and blessings be upon him) and said: "O Prophet of Allah, Mu'adh bin Jabal (may Allah be pleased with him) told me that you said: 'Whoever bears witness with sincerity that there is no true god except Allah will enter Paradise.'" He (peace and blessings be upon him) said: "Mu'adh has spoken the truth, Mu'adh has spoken the truth, Mu'adh has spoken the truth."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17216