«المسلم إذا سئل في القبر يشهد أن لا إله إلا الله وأن محمدا رسول الله فذلك قوله تعالى: {يُثَبِّتُ اللَّهُ الَّذِينَ آمَنُوا بِالْقَوْلِ الثَّابِتِ فِي الْحَيَاةِ الدُّنْيَا وَفِي الْآخِرَةِ} »
The Prophet (peace and blessings be upon him) said:
"When a Muslim is questioned in the grave, he testifies 'La ilaha illallah wa anna Muhammadan rasulullah' (There is no true god except Allah, and Muhammad (peace and blessings be upon him) is the Messenger of Allah), and this is the meaning of the saying of Allah Almighty: Allah Almighty keeps firm those who believe with the firm word in the life of this world and in the Hereafter."
Hadith Referenceصحیح الجامع الصغیر / حرف الميم / 6708