Sahih al-Bukhari 4699
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · "Allah will keep firm those who believe with, the word that stands firm..." (V.14:27)
Narrated Al-Bara bin Azib:
حَدَّثَنَا أَبُو الْوَلِيدِ ، حَدَّثَنَا شُعْبَةُ ، قَالَ أَخْبَرَنِي عَلْقَمَةُ بْنُ مَرْثَدٍ ، قَالَ سَمِعْتُ سَعْدَ بْنَ عُبَيْدَةَ ، عَنِ الْبَرَاءِ بْنِ عَازِبٍ ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم 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} "
Allah's Messenger (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 Allah's Messenger (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)