كتاب الإيمان
1
The Book of Faith
(9)
Chapter: Evidence that the islam of one who becomes muslim on his deathbed is valid, so long as the death thoes have not begun; Abrogation of permission to supplicate for forgiveness for the idolators; Evidence that one who dies an idolator is one of the people of hell and no intervention can save him from that
(9)
باب الدَّلِيلِ عَلَى أَنَّ مَنْ مَاتَ عَلَى التَّوْحِيدِ دَخَلَ الْجَنَّةَ قَطْعًا

It is narrated on the authority of Abu Huraira that the Messenger of Allah said to his uncle at the time of his death:

Make a profession of it that there is no god but Allah and I will bear testimony (of your being a Muslim) on the Day of judgment. But he (Abu Talib) refused to do so. Then Allah revealed this verse: " Verily thou canst not guide to the right path whom thou lovest. And it is Allah Who guideth whom He will and He knoweth best who are the guided" (xxviii. 56).
حَدَّثَنَا مُحَمَّدُ بْنُ عَبَّادٍ، وَابْنُ أَبِي عُمَرَ، قَالاَ حَدَّثَنَا مَرْوَانُ، عَنْ يَزِيدَ، - وَهُوَ ابْنُ كَيْسَانَ - عَنْ أَبِي حَازِمٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم لِعَمِّهِ عِنْدَ الْمَوْتِ ‏"‏ قُلْ لاَ إِلَهَ إِلاَّ اللَّهُ أَشْهَدُ لَكَ بِهَا يَوْمَ الْقِيَامَةِ ‏"‏ ‏.‏ فَأَبَى فَأَنْزَلَ اللَّهُ ‏{‏ إِنَّكَ لاَ تَهْدِي مَنْ أَحْبَبْتَ‏}‏ الآيَةَ ‏.‏
Reference : Sahih Muslim 25 a
In-book reference : Book 1, Hadith 41
USC-MSA web (English) reference : Book 1, Hadith 37
  (deprecated numbering scheme)