Musnad Ahmad 3:49

'Abd al-Wahhab al-Khaffaf narrated to us, Sa'id narrated to us, from Qatadah, from Muslim ibn Yasar, from Humran ibn Aban, that 'Uthman ibn 'Affan said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "I surely know a word that no servant says truthfully from his heart except that he is forbidden to the Fire." 'Umar ibn al-Khattab said to him: "Shall I tell you what it is? It is the word of sincerity (al-ikhlas) which Allah, Blessed and Exalted is He, bound Muhammad, peace and blessings of Allah be upon him, and his companions to. It is the word of piety (al-taqwa) which the Prophet of Allah, peace and blessings of Allah be upon him, urged his uncle Abu Talib to accept at the time of death: the testimony that there is no god but Allah."