Sahih Bukhari Volume 2, Book 23, Hadith Number 450.

Narated By 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).