Midrash su Salmi 68:24

Rabbi Eliezer said: || On the day when He said, "Let the waters be gathered together" (Gen. 1:9), on that very day were the waters congealed, and they were made into twelve valleys, corresponding to the twelve tribes, and they were made into walls of water between each path, and (the people) could see one another, and they saw the Holy One, blessed be He, walking before them, but the heels of His feet they did not see, as it is said, "Thy way was in the sea, and thy paths in the great waters, and thy footsteps were not known" (Ps. 77:20).