يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا بِالْقَوْلِ الثَّابِتِ
Allah keeps the believers firm with the firm word.
Questioning in the grave concerns the interim life: the person is questioned about Lord, religion and Prophet. Its reality is affirmed through revelation and reports, not reduced to the ordinary conditions of worldly observation.
Resurrection on the Last Day is a distinct later stage. Grave questioning, the intermediate state, rising again and the Reckoning are connected but not identical events.