Tafsir of Al-Kahf 18:64

"He said," that is, Moses, peace be upon him, "that," which I mentioned regarding the matter of the fish, "is what we were seeking," meaning that which we were searching for, in that it was a sign of attaining what is essentially desired.

It is read as nabghi (نبغ) without the ya (ي) in wasl (continuous recitation), and preserving it is better; it is the reading of Abu ‘Amr, al-Kisa’i, and Nafi’. As for stopping (waqf), the majority drop the ya in accordance with the script of the Mus’haf, while Ibn Kathir preserves it in both states.

"So they returned," meaning they turned back, "upon their footsteps," the first ones, and the intended meaning is the path they had come from.

"Tracing," meaning they were following them closely. It is from the expression "to trace the footprint," when one follows it, as is apparent. Its accusative case is because it is the object of an implied verb derived from the same root. It is also permissible that it is a circumstantial qualifier (hal) interpreted as an attribute, meaning "following the path," until they reached the rock where they had lost the fish.