Tafsir of At-Takweer 81:26

"So where are you going?" is a way of declaring their straying in the path they have taken regarding the matter of the Great Quran. It is like your saying to someone who has abandoned the main road and is traveling along the narrow paths, "Where are you going?" The fa (so) is for connecting what follows to what precedes it, given that it is established that it is a revelation.