Midrasz do Hioba 1:11

Then came Amalek and fought with Israel in Rephidim (Exod. 17:8). R. Eliezer the son of Hisma said: This verse should be understood and interpreted in relation to the verse quoted in Job: Can the rush shoot up without mire? Can the reed grass grow without water? (Job 1:11). Just as that is impossible, so is it impossible for Israel to survive unless the people devote themselves to the law. It was because Israel turned away from the law that the enemy attacked them. Indeed, the enemy attacked them only as a consequence of their sins and transgressions. Hence it says: Then came Amalek.