Tafsir of Sad 38:84

(He said, "Then the truth, and the truth I speak"): The first is in the nominative case, either as a subject whose predicate is omitted or as a predicate whose subject is omitted. The second is in the accusative case as the object of what follows it, having been advanced for the purpose of restriction—meaning: "I speak nothing but the truth." The fa (then) is for the sequence of the content of what follows it upon what precedes it; meaning: "Then the truth is My oath."