Parashas Beshalach · Passuk 15:22
Chabad.org — English translation + Rashi
Torah Temimah points to Bava Kama 82a (verified).
English: The Dorshei Reshumot said: “Water” means nothing other than Torah, as it is stated “Ho, everyone who is thirsty, go to the water.” Once they had gone three days without Torah they became weary; the prophets among them arose and instituted that they should read on Shabbos, Monday, and Thursday.
In context: See the cited source for the full section.
Derash on the verse.
The three days without water allude to three days without Torah; the prophets therefore established the public reading of the Torah on Mondays, Thursdays, and Shabbos.