When a Muslim is questioned in the grave he testifies that there is no god but Allah and that Muhammad is Allah’s Apostle. That is verified by Allah’s words: “Allah establishes those who believe with the word that stands firm.”
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِنَّ الْمُسْلِمَ إِذَا سُئِلَ فِي الْقَبْرِ فَشَهِدَ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ ﷺ فَذَلِكَ قَوْلُ اللَّهِ ﷻ { يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا بِالْقَوْلِ الثَّابِتِ }