"And the waters were a wall to them" (Exodus 14:22): When Israel went down into the sea, Gabriel went down with them and encircled them and guarded them like a wall. And he would proclaim within the waters, to the right: Take care with Israel, for they are destined to receive the Torah from the right hand of the Holy One, blessed be He. And to the left he would say: Take care with these, for they are destined to lay tefillin on the left. And to the waters before them he would say: Take care with these, for they are destined to seal themselves before them in the covenant. And to those behind them he would say: Take care with these, for they are destined to display the knot of the tefillin and the corner of their fringes behind them.
Gabriel Proclaims Israel's Future Mitzvot Within the Sea
Yalkut Shimoni on Torah 234:5
וְהַמַּיִם לָהֶם חוֹמָה, בְּשָׁעָה שֶׁיָּרְדוּ יִשְׂרָאֵל לַיָּם יָרַד גַּבְרִיאֵל עִמָּהֶם וְהִקִּיפָם וּשְׁמָרָם כַּחוֹמָה, וְהָיָה מַכְרִיז (בֵּין מַיִם לְמַיִם) [בַּמַּיִם, לְיָמִין], הִזָּהֲרוּ בְּיִשְׂרָאֵל שֶׁעֲתִידִין לְקַבֵּל הַתּוֹרָה מִימִינוֹ שֶׁל הַקָּדוֹשׁ בָּרוּךְ הוּא. וְלִשְׂמֹאל, הָיָה אוֹמֵר, הִזָּהֲרוּ בְּאֵלּוּ שֶׁעֲתִידִין לְהָנִיחַ תְּפִלִּין בִּשְׂמֹאל. וְלַמַּיִם שֶׁלִּפְנֵיהֶם הָיָה אוֹמֵר, הִזָּהֲרוּ בְּאֵלּוּ שֶׁעֲתִידִין שֶׁיְּהוּ חוֹתְמִין לִפְנֵיהֶן בַּבְּרִית. וְשֶׁלְּאַחֲרֵיהֶם הָיָה אוֹמֵר, הִזָּהֲרוּ בְּאֵלּוּ שֶׁעֲתִידִין לְהַרְאוֹת קֶשֶׁר שֶׁל תְּפִלִּין וּכְנַף צִצִּיּוֹתֵיהֶן מֵאַחֲרֵיהֶן.