Musannaf Ibn Abi Shayba 33:543

Abu Mu'awiyah narrated to us, from al-A'mash, from Sa'd ibn 'Ubaydah, from al-Bara' ibn 'Azib, who said: Regarding His saying: "Allah keeps firm those who believe, with the firm word, in the worldly life," he said: The firmness in the worldly life is when the two angels come to the man in the grave and say to him: "Who is your Lord?" He says: "My Lord is Allah." They say: "What is your religion?" He says: "My religion is Islam." They say: "And who is your Prophet?" He says: "Muhammad." He said: That is the firmness in the worldly life.