Musnad Ahmad 252
It was narrated that ash-Sha`bi said:
`Umar passed by Talhah, and he narrated a similar report. He said: ‘Umar passed by Talhah and saw him looking worried. He said: Perhaps you are upset because your cousin was appointed (as caliph)? - referring to Abu Bakr. He said: No, but by Allah, I heard the Messenger of Allah (ﷺ) say: `I know a word which, if a man says it when he is dying, it will be light in the record of his deeds, or he will find comfort in it at the time of death.` ‘Umar said: I will tell you of it; it is the word that he wanted his paternal uncle to say, the testimony that there is no god but Allah. He said: It is as if a great burden has been lifted from me. He said: You are right. If he had known anything better that it, he would have instructed him to say it.
حَدَّثَنَا يَحْيَى، عَنْ إِسْمَاعِيلَ، حَدَّثَنَا عَامِرٌ، و حَدَّثَنَا مُحَمَّدُ بْنُ عُبَيْدٍ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ أَبِي خَالِدٍ، عَنْ رَجُلٍ، عَنِ الشَّعْبِيِّ، قَالَ مَرَّ عُمَرُ بِطَلْحَةَ فَذَكَرَ مَعْنَاهُ قَالَ مَرَّ عُمَرُ بِطَلْحَةَ فَرَآهُ مُهْتَمًّا قَالَ لَعَلَّكَ سَاءَكَ إِمَارَةُ ابْنِ عَمِّكَ قَالَ يَعْنِي أَبَا بَكْرٍ رَضِيَ اللَّهُ عَنْهُ فَقَالَ لَا وَلَكِنِّي سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ إِنِّي لَأَعْلَمُ كَلِمَةً لَا يَقُولُهَا الرَّجُلُ عِنْدَ مَوْتِهِ إِلَّا كَانَتْ نُورًا فِي صَحِيفَتِهِ أَوْ وَجَدَ لَهَا رَوْحًا عِنْدَ الْمَوْتِ قَالَ عُمَرُ أَنَا أُخْبِرُكَ بِهَا هِيَ الْكَلِمَةُ الَّتِي أَرَادَ بِهَا عَمَّهُ شَهَادَةُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ قَالَ فَكَأَنَّمَا كُشِفَ عَنِّي غِطَاءٌ قَالَ صَدَقْتَ لَوْ عَلِمَ كَلِمَةً هِيَ أَفْضَلُ مِنْهَا لَأَمَرَهُ بِهَا‏.‏