Midrasch zu Tehillim 76:1

Rabbi 'Aḳiba said: The Israelites advanced to enter the Reed Sea, but they turned backwards, fearing lest the waters would come over them. The tribe of Judah sanctified His great Name, and entered the sea first, and under the dominion of the hand likewise of the sons of Judah (did all Israel enter the sea after them), as it is said, "Judah became his sanctuary, Israel his dominion" (Ps. 114:2). The Egyptians desired to follow after Israel, but they turned backwards, fearing lest the waters would return over them. What did the Holy One, blessed be He, do? He appeared before them like a man riding on the back of a mare, as it is said, "To a steed in Pharaoh's chariots" (Cant. 1:9). The horse on which Pharaoh rode saw the mare (of God), and it neighed and ran and entered the sea after it.