Sahih Muslim Book 40, Hadith Number 6865.

Chapter : The dead would be shown his seat in Paradise and hell, and the affirmation of the torment of the grave and seeking refuge from it.

Al-Bara’ b. ‘Azib reported Allah’s Apostle (may peace be upon him) 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 (may peace be upon him), 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.”