Exodus 17:8

Then, i.e. when they were upon their march from Rephidim to Horeb, Deuteronomy 25:17,18. The ground of the quarrel was the prosecution of the old hatred of Esau a against Jacob, and-the revenging of themselves and their father upon the posterity of Jacob; for which they thought this the fittest season, they being now great and potent people, Numbers 24:20, and Israel now weak, and unarmed, and dispirited with long servitude.