Tafsir of Ash-Shu`ara' 26:52

This occurred after he had remained among them for years, calling them to the Truth and demonstrating the signs to them, yet they increased only in insolence and obstinacy, as detailed in Surah al-A'raf by the saying of the Exalted, "And We certainly seized the people of Pharaoh with years [of famine]..." and the following verses.

It is read as an-isri (أنِ اسرِ), with a kasra on the nun and a connected alif, derived from sara (to travel by night). Al-Yamani read it as in sir (إن سِر), as an imperative from sara-yasiru (to march).

"...for you will be pursued."

This is the justification for the command to set out by night; that is, Pharaoh and his soldiers will follow you in the morning. Therefore, set out at night with those who are with you so that they do not overtake you before you reach the sea. Rather, they will be on your trail when you enter the sea, so they will enter where you entered, and I shall close it upon them and drown them.