MIDRASH 1

IN THE BEGINNING (BE-RESHITH) GOD CREATED. R. Jonah said in R. Levi's name: Why was the world created with the letter bet? Just as the bet is closed at the sides but open in front, so you are not permitted to investigate what is above and what is below, what is before and what is behind.

-Midrash Rabbah, Genesis I:10