Sahih al-Bukhari 1369
Narrated Al-Bara' bin 'Azib :
The Prophet (p.b.u.h) said, "When a faithful believer is made to sit 
     in his grave, then (the angels) come to him and he testifies that none
     has the right to be worshipped but Allah and Muhammad is Allah's 
     Apostle. And that corresponds to Allah's statement: Allah will keep 
     firm those who believe with the word that stands firm . . . (14.27). 



     Narrated Shu'ba:


     Same as above and added, "Allah will keep firm those who believe . . .
     (14.27) was revealed concerning the punishment of the grave."
حَدَّثَنَا حَفْصُ بْنُ عُمَرَ، حَدَّثَنَا شُعْبَةُ، عَنْ عَلْقَمَةَ بْنِ مَرْثَدٍ، عَنْ سَعْدِ بْنِ عُبَيْدَةَ، عَنِ الْبَرَاءِ بْنِ عَازِبٍ ـ رضى الله عنهما ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏"‏ إِذَا أُقْعِدَ الْمُؤْمِنُ فِي قَبْرِهِ أُتِيَ، ثُمَّ شَهِدَ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ، وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ، فَذَلِكَ قَوْلُهُ ‏{‏يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا بِالْقَوْلِ الثَّابِتِ‏}‏ ‏"‏‏.‏ حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ حَدَّثَنَا غُنْدَرٌ حَدَّثَنَا شُعْبَةُ بِهَذَا وَزَادَ ‏{‏يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا‏}‏ نَزَلَتْ فِي عَذَابِ الْقَبْرِ‏.‏