Tafsir of Yunus 10:76

فلما جاءهم الحق من عندنا قالوا إن هذا لسحر مبين

So, when came to them the truth from us, they said: "This is indeed clear magic.'' They were as if they gave an oath that what they had said was the truth. But they knew that what they were saying was a mere lie. As Allah said:

وجحدوا بها واستيقنتها أنفسهم ظلما وعلوا

And they belied them wrongfully and arrogantly, though they themselves were convinced thereof. (27:14) Musa criticized them saying:

قال موسى أتقولون للحق لما جاءكم أسحر هذا وﻻ يفلح الساحرون