قَالَ لَهُم مُّوسَىٰ وَيْلَكُمْ لَا تَفْتَرُوا۟ عَلَى ٱللَّهِ كَذِبًا فَيُسْحِتَكُم بِعَذَابٍ ۖ وَقَدْ خَابَ مَنِ ٱفْتَرَىٰ Qur’an Ta-Ha (20:61)Qala lahum moosa waylakum la taftaroo AAala Allahi kathiban fayushitakum biAAathabin waqad khaba mani iftara
I.e., by deliberately denying the truth of His messages.
By saying that Moses’ miracle is pure magic.
Moses had some idea of their trickery and deceit. They would palm off their fraudulent magic as coming from Allah or from their gods! He warns them that their tricks will stand exposed, and their hopes will be defeated.