From Sahih Muslim (Hadith 7219):
Al-Bara' b. `Azib reported Allah's Apostle (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
Reference: Muslim 7219