Al-Bara' b. `Azib reported Allah's Messenger (ﷺ) as saying: This verse: "Allah grants steadfastness to those who believe with firm word," was revealed in connection with the torment of the grave. It would be said to him: Who is your Lord? And he would say: Allah is my Lord and Muhammad is my Apostle (ﷺ), and that is (what is implied) by the words of Allah, the Exalted: "Allah keeps steadfast those who believe with firm word in this world and in the Hereafter."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارِ بْنِ عُثْمَانَ الْعَبْدِيُّ، حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ عَلْقَمَةَ بْنِ مَرْثَدٍ، عَنْ سَعْدِ بْنِ عُبَيْدَةَ، عَنِ الْبَرَاءِ بْنِ عَازِبٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " { يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا بِالْقَوْلِ الثَّابِتِ} قَالَ " نَزَلَتْ فِي عَذَابِ الْقَبْرِ فَيُقَالُ لَهُ مَنْ رَبُّكَ فَيَقُولُ رَبِّيَ اللَّهُ وَنَبِيِّيَ مُحَمَّدٌ صلى الله عليه وسلم . فَذَلِكَ قَوْلُهُ عَزَّ وَجَلَّ { يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا بِالْقَوْلِ الثَّابِتِ فِي الْحَيَاةِ الدُّنْيَا وَفِي الآخِرَةِ} " .
sunnah.com