Sahih Muslim Book 53, Hadith 89
"Allah keeps those who believe steadfast with firm word in this world and the Hereafter" was revealed in connection with the torment of the grave.
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."
When the soul of a believer would go out (of his body) it would be received bv two angels who would take it to the sky. Hammad (one of the narrators in the chain of transmitters) mentioned the swetness of its odour, (and further said) that the dwellers of the sky say: Here comes the pious soul from the side of the earth Let there be blessings of Allah upon the body in which it resides. And it is carried (by the angels) to its Lord, the Exalted and Glorious. He would say: Take it to its destined end. And if he is a nonbeliever and as it (the soul) leaves the body-Hammad made a mention of its foul smell and of its being cursed-the dwellers of the sky say: There comes a dirty soul from the side of the earth, and it would be said: Take it to its destined end. Abu Huraira reported that Allah's Messenger (ﷺ) put a thin cloth which was with him upon his nose while making a mention (of the foul smell) of the soul of a non-believer.