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.” (Using translation from Abū Dāʾūd 4750)
("إِنَّ الْمُؤْمِنَ إِذا شهد أَن لَّا إِله إِلا اللهُ، وعَرَفَ مُحمَّدًا في قَبْرِه فَذَلِكَ قوله تَعَالى: {يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا بِالْقَوْلِ الثَّابِتِ ... } الآية ".