TT #45 · Parashas Va’eschanan · Passuk 6:6
And these words that I command you today shall be upon your heart.
Torah Temimah points to the requirement that the paragraphs of Shema (and of tefillin / mezuzah) must be written and read in their proper order — “vehayu be-havayyasam” (Menachos 32a / parallel).
English: The Sages taught: “And they shall be” — they shall be in their [proper] being, so that one not read them out of order.
Diyuk of “vehayu” as requiring the original order of the words and paragraphs.
The paragraphs of Shema (and the texts of tefillin and mezuzah) must be written and recited in their proper order; reading or writing them backwards is invalid.