Tafsir of Ad-Dukhan 44:23

(So set out with My servants)—who are the Children of Israel and those who believed in him from among the Copts—(by night)—meaning during a portion of the night. The speech contains an elliptical omission of a verb of saying after the fa (so), meaning: "He said: Set out, etc." Thus, the fa is for sequence and ordering, and the verb of saying is linked to what preceded it or what came before that. It is as if it were said: "He said," or "He said, if the matter is as you say: So set out, etc."

The fa is positioned in the answer to an implied conditional, and it is the answer to the speech that is implied, whether with the fa or without it, assuming it is an initiation of speech. The first instance of omission is more appropriate due to the paucity of required insertions, especially since the assumption of a conditional does not fit here, as there is no doubt in the matter, neither in reality nor in the context of the assumption. Treating it as having the meaning of "when" (idha) is an over-burdened interpretation. Abu Hayyan does not permit the deletion of the condition while keeping its answer in such a context, and he has criticized Al-Zamakhshari for allowing it. Nafi' and Ibn Kathir read fa-asri with a connected hamza (from sara).

(Indeed, you will be pursued)—Pharaoh and his soldiers will follow you once they learn of your departure. Thus, the sentence is an initiation of speech to provide the reason for the command to set out by night, so that the knowledge of it is delayed and they do not overtake you. The emphasis is used because of the preceding [context] which intimates the news.