Rashi on Genesis 48:13/רש"י על בראשית מ״ח:י״ג

1את אפרים בימינו משמאל ישראל EPHRAIM ON HIS RIGHT HAND TOWARDS ISRAEL’S LEFT — If one is moving towards another person his right hand is opposite the other’s left. Now since he (Manasseh) was the first-born he was marked out for the blessing and should have been placed at Jacob’s right hand.