I heard the Messenger of Allah ﷺ say: ʿI know a word which no one says, sincerely from the heart, but he will be forbidden to the Fire.” ʿUmar bin al-Khattab ؓ said to him: I will tell you what it is: it is the word of al-ikhlas by means of which Allah, may He be blessed and exalted, caused Muhammad ﷺ and his companions to prevail and it is the word of taqwa that the Prophet ﷺ of Allah urged his uncle Abu Talib to say when he was dying, the testimony that there is no god but Allah. (Using translation from Aḥmad 447)
سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ «إِنِّي لَأَعْلَمُ كَلِمَةً لَا يَقُولُهَا عَبْدٌ حَقًّا مِنْ قَلْبِهِ فَيَمُوتُ إِلَّا حُرِّمَ عَلَى النَّارِ» فَقُبِضَ رَسُولُ اللَّهِ ﷺ وَلَمْ يُخْبِرْنَاهَا فَقَالَ عُمَرُ بْنُ الْخَطَّابِ أَنَا أُخْبِرُكَ بِهَا هِيَ كَلِمَةُ الْإِخْلَاصِ الَّتِي أَمَرَ بِهَا رَسُولُ اللَّهِ ﷺ عَمَّهُ أَبَا طَالِبٍ عِنْدَ الْمَوْتِ شَهَادَةُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَهِيَ الْكَلِمَةُ الَّتِي أَكْرَمَ اللَّهُ بِهَا مُحَمَّدًا وَأَصْحَابَهُ