Tafsir of Al-Qasas 28:36

{سحر مفترى} (A fabricated magic): Magic that you perform yourself and then falsely attribute to God. Or, magic whose fabrication is manifest. Or, it is described as a fabrication like all other types of magic, and is not a miracle from God.

{وإذا رأيت} (And when you saw): An accusative state (ḥāl) referring to this [claim], meaning: existing in their time and days.

He means: What has been narrated to us regarding their state. It is inevitable that they are lying in this, for they have heard and known of the like of it. Or, he means that they had never heard of anything like it in its enormity. Or, it refers to what the soothsayers used to foretell regarding the appearance of Moses and his coming with what he brought.

This is evidence that they were refuted and confounded, finding no way to repel the signs he brought them except by saying: "This is magic and an innovation, the like of which we have never heard."

{وقال موسى ربي أعلم بمن جاء بالهدى من عنده ومن تكون له عاقبة الدار إنه لا يفلح الظالمون} (And Moses said, "My Lord is most knowing of who has brought guidance from Him and to whom will be the [good] outcome of the home. Indeed, wrongdoers do not succeed.")