Musar zu Schemot 20:10

לא תעשה כל מלאכה. There are thirty nine categories of work which are forbidden on the Sabbath. Rabbi Menachem Habavli writes that these categories correspond to the types of work the Israelites had to perform for the Egyptians during their years of bondage, i.e. the nnfl nnap mentioned in Exodus 1,13. When you look at the letters in the word עבודת פרך by using a method called את,בש,גר,דק, etc., you will find that the letters פ-ר-ך when exchanged with each other according to this method, when added together amount to 39. [compare Sanhedrin 22 on how Daniel deciphered the writing on the wall of Belshazzar's palace by using this method. Ed.] Thus far Rabbi Menachem Habavli. The Sabbath and the Exodus are part of the same utterance by G–d, as I have explained. The mystical dimension of the "private domain" on the Sabbath is that it must be a minimum of י (10) טפחים high and occupy an area of not less than ד (4 by 4) טפחים. 4 by 4 plus 10 equals the numerical value of the Ineffable Name י-ה-ו-ה=26.