Midrash su Sukkah 4:13

Rabban Gamaliel said: The Egyptians pursued after the children of Israel as far as the Reed Sea, and encamped behind them. The enemy was behind them and the sea || was in front of them. And the Israelites saw the Egyptians, and feared very greatly, and there they cast away from themselves all the Egyptian abominations, and they repented very sincerely, and called upon their God, as it is said, "And when Pharaoh drew nigh, the children of Israel lifted up their eyes" (Ex. 14:10). Moses beheld the anguish of Israel, and arose to pray on their behalf. The Holy One, blessed be He, said to him: "Speak unto the children of Israel, that they go forward" (Ex. 14:15).