Mustadrak al-Hakim — Hadith 1314

Sayyiduna Uthman bin Affan (may Allah be pleased with him), relating a hadith to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "I know a word which no servant says truly from his heart and then dies, except that the Fire (of Hell) is made forbidden for him." But the Messenger of Allah (peace and blessings be upon him) passed away without having told us (clearly) about that word. Umar bin al-Khattab (may Allah be pleased with him) said: I will tell you about it; it is the word of sincerity (kalimat al-ikhlas) which the Messenger of Allah (peace and blessings be upon him) commanded his uncle Abu Talib to say at the time of (Abu Talib's) death, namely bearing witness that "there is no god but Allah," and it is the word by which Allah honoured Muhammad (peace and blessings be upon him) and his Companions.
This hadith is sahih (authentic) on the condition of the two Shaykhs, but they did not record it with this wording; rather, Imam Muslim alone recorded the narration of Khalid al-Hadhdha' from al-Walid bin Muslim that the Prophet (peace and blessings be upon him) said: "Whoever dies knowing that there is no god but Allah will enter Paradise."