Midrash su Salmi 74:11

(Shemoth 13:9) "upon your hand" — the left hand, as it is written (Isaiah 48:13) "My hand also has founded the earth, and My right hand has spanned the heavens," and (Judges 5:26) "She sent forth her hand for the (tent-) peg and her right hand for the laborers' hammer," and (Psalms 74:11) "Why do You withdraw Your hand, and Your right hand, etc." — whence we see that "hand" (by itself) in all places is the left hand.