Sefer HaBahir 14

1[Rabbi Bun continued,] "Why is [the letter] Bet closed on all sides and open in the front? To teach you that it is the house [bayit] of the world and that the Blessed Holy One, is the place of the world yet the world is not His place. Don’t read 'Bet', rather 'bayit'. This what is written, 'With wisdom [בחכמה], a house is built, and with understanding [בתבונה], it is established' (Proverbs 24:3).”