Al-Bara ibn Azib reports that the Messenger of Allah ﷺ explained “Allah keeps the believers firm through the firm word” in connection with questioning and punishment in the grave. (Sahih Muslim, Book of Paradise, chapter on the deceased person's place)
When asked “Who is your Lord?”, the believer answers, in meaning, “My Lord is Allah, and my Prophet is Muhammad ﷺ.”
The firm response is connected with the verse's promise: faith in life becomes firmness in the questioning of the grave.