Midrash su Esodo 38:1

Another explanation of the word shittim. The shin stands for shalom (“peace”), the tet for tov (“good”), the yod for yeshuah (“salvation”), and the mem for mehilah (“pardon”). They held the tablets on which the ten words (commandments) are written; five were inscribed on one tablet, and five on the other. Five cubits was the length thereof, and five cubits the breadth thereof (Exod. 38:1). They contained the tablets, for it is written concerning the Ten Commandments, five were on one tablet and five on the other. And the height thereof shall be three cubits (ibid. 27:1). This corresponds to the three redeemers, as is said: I sent before thee Moses, Aaron, and Miriam (Mic. 6:4).