שמות יג ,ט : וְהָיָה לְךָ לְאוֹת עַל־יָדְךָ
The Baal haTurim states that the Gematria of the entire phrase (647) is equal to that of “zera shmol” (the left arm). Chaza”l uses many “proofs” that one place Tefillin on the left (weak) arm, and this is another hint.