Tafsir of An-Nazi'at 79:22

( Then he turned back ) means he turned away from obedience, ( striving ) meaning he was exerting himself and laboring to invalidate his [Musa's] affair, peace be upon him, and to oppose the sign. The use of "then" (thumma) is because the invalidation and overturning of that requires a long duration of time. It has been permitted that the turning away (idbar) refers to its literal meaning, that is, he turned away from the assembly, striving to invalidate that.

It is said that "turned back, striving" means he fled from the serpent; for it is narrated that when he [Musa] cast the staff, it transformed into a terrifying serpent, opening its jaws with eighty cubits between its upper and lower jaws. It placed its lower jaw on the ground and its upper jaw upon the palace wall. Pharaoh fled, defecated in his clothes, and the people fled in a stampede, resulting in the death of twenty-five thousand of his people.

In some traditions, it is mentioned that it transformed into a snake and ascended into the sky the distance of a mile, then descended, heading toward Pharaoh and saying, "O Musa, command me as you wish." Pharaoh said, "I adjure you by the One who sent you to take it," so he took it and it returned to being a staff.

You know that if this occurred after the gathering of the sorcerers for opposition—as is the famous account—the validity of intending it here does not appear clear if the gathering intended is the one that occurred after their [initial] assembly. And if it were after the denial and disobedience but before the gathering, its delay (tarakhi) relative to the former acts is not apparent. Yes, it has been said that "then" (thumma) is used here to indicate the implausibility of his turning back—terrified and hastening—despite his claim to divinity.

It is also said that the intended meaning of His saying—Glory be to Him—"Then he turned back, then he proceeded" is based on the expression "he proceeded to do such-and-such," meaning he began [the act]; however, the turning away (idbar) was placed in the position of proceeding (iqbal) as a form of wit and an alerting to the fact that his state was one of destruction and retreat.