Midrash sobre Zacarias 2:5

Moses spake before the Holy One, blessed be He, saying: Sovereign of all worlds ! The enemy is behind them, and the sea is in front of them, which way shall they go forward? What did the Holy One, blessed be He, do? He sent Michael, and he became a wall of fire between (Israel and) the Egyptians. The Egyptians desired to follow after Israel, but they are unable to come (near) because of the fire. The angels beheld the misfortune of Israel all the night, and they uttered neither praise nor sanctification to their Creator, as it is said, "And the one came not near the other all the night" (Ex. 14:20).

Rabbi Azariah said: Amalek was a descendant of Esau, and because of his ancestor's enmity he came against them to punish them. The cloud was surrounding the camp of Israel like a city surrounded by a wall. The adversary and enemy were unable to touch them, but (when) anyone needed a ritual bath the cloud excluded him from the camp of Israel, because the camp of Israel was holy, as it is said, "Therefore shall thy camp be holy" (Deut. 23:14), and (then) Amalek was smiting and slaying the hindmost of those who were beyond the cloud, as it is said, "And he smote the hindmost of thee, all that were feeble behind thee" (Deut. 25:18).