«فَلَا تَثِقْ بِوَمِيضٍ مِنْ بَرْقِهِ فَهُوَ خُلَّبُ»
Do not trust the flash of its lightning, for it may be deceptive.
Wamīḍ is a brief gleam; barq is lightning; khullab describes a flash that suggests rain but fails to bring the hoped-for benefit.
Literally, the image is a flash of lightning. Figuratively, it warns against trusting every attractive promise of worldly fortune. Lā tathiq is a negative command, giving the image the force of direct counsel.