﴿ فَلَمَّا كَشَفْنَا عَنْهُمُ ٱلرِّجْزَ إِلَىٰٓ أَجَلٍ هُم بَٰلِغُوهُ إِذَا هُمْ يَنكُثُونَ ﴾

“But whenever We removed the plague from them, giving them time to make good their promise, lo, they would break their word.”

(But when We did remove from them the terror) the punishment (for a term which they must reach) i.e. the drowning, (behold! they broke their covenant) they broke their pledge to Moses.