Messenger of Allah ﷺ said, "When a Muslim is questioned in his grave, he will testify that none has the right to be worshipped but Allah and that Muhammad is Messenger of Allah ﷺ, and that is what is meant by Allah's Statement:-- "Allah will keep firm those who believe with a Word that stands firm in this world and in the Hereafter." (14.27)
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ الْمُسْلِمُ إِذَا سُئِلَ فِي الْقَبْرِ يَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ فَذَلِكَ قَوْلُهُ {يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا بِالْقَوْلِ الثَّابِتِ فِي الْحَيَاةِ الدُّنْيَا وَفِي الآخِرَةِ}