Tafsir of As-Saffat 37:81

(Indeed, he was of Our believing servants)

This is the justification for his—peace be upon him—being among the "doers of good" (muhsinin), which is understood from the discourse by the sincerity of his servitude and the perfection of his faith. Therein lies an indication of the magnificence of their status that is not hidden; otherwise, the station of messengership is a great station, and a messenger is never devoid of sincerity in servitude and perfection of faith. Thus, the intent behind the description is the praise of the attribute itself, not the praise of the one being described.