Tehilim 136:13 says:
לְגֹזֵר יַם סוּף לִגְזָרִים כִּי לְעוֹלָם חַסְדּוֹ
To Him Who cut the Sea of Reeds asunder, for His kindness is eternal.
Rashi's commentary indicates that this is talking about God cutting twelve paths through the sea, one for each tribe. Does anyone know what Rashi's source in the Midrash is, and where this idea shows up for the first time in the Midrash?
This question was adapted from a Facebook post by DovBear.