Rashi on Exodus 13:9/רש"י על שמות י״ג:ט׳

1והיה לך לאות AND IT SHALL BE FOR A SIGN UNTO THEE — i. e. the Exodus from Egypt shall be to thee as a sign

2על ידך ולזכרון בין עיניך UPON THINE HAND AND AS A MEMORIAL BETWEEN THINE EYES — meaning, that you shall write these paragraphs (verses 1—10 and 11—16) and bind them upon the head and upon the arm.

3על ידך UPON THY HAND — on the left hand; that is why the word is written יָדְכָה in its full form (not יָדְךָ) in the second section (v. 16), so that we should explain it (by dividing this word into two: יד כה) as meaning the hand that is weak (כהה) (Menachot 36b).