قَالَ الرَّسُولُ: أَقَامُوا فِي الْوَادِي. كَانَ الضَّبَابُ شَدِيدًا.
The messenger said: They stayed in the valley. The fog was dense.
The first clause can raise the unspoken question, “Why did they remain there?” The second supplies a contextual explanation, making explanatory resumption a credible reading. The absence of wāw supports that implied question-and-answer relation.
If the purpose were merely to list separate observations, the discourse relation could differ. Istiʾnāf is not a label automatically attached to every second sentence.