Rashi, on Gen 49:3

“Ruben, my firstborn, thou art my strength, and the beginning of my sorrow: excelling in gifts, greater in command.”

“אוני means MY VIGOUR, as (Hosea 12:9) “I have gained me (און) strength”; (Isaiah 40:26) “by the greatness of His (אונים) might”; (Isaiah 5:29) “and to him that hath no (אונים) might Ho increaseth strength”. יתר שאת SUPERIORITY IN DIGNITY — Since you were the first-born it was proper that you should be superior to your brothers by being endowed with the priesthood. This term שאת “lifting up” alludes to the נשיאות כפים the raising of the hands when the priests pronounce the benediction (Genesis Rabbah 98:4; cf. Onkelos). ויתר עז SUPERIORITY IN POWER — meaning in royal rank, as is meant by עז in (1 Samuel 2:10) “And he will give strength (עז) unto His king”. And what caused you to lose all these?”

Imported from an open dataset — not yet checked against the printed edition.