Rashi on Deuteronomy 3:18/רש"י על דברים ג׳:י״ח

1ואצו אתכם AND I COMMANDED YOU — He now addresses himself to the sons of Reuben and the sons of Gad.

2לפני אחיכם [YE SHALL PASS OVER EQUIPPED] BEFORE YOUR BRETHREN — they used to go in front of the other Israelites into battle because they were mighty men, and the enemy fell before them, as it is said of Gad, (Deuteronomy 33:20) "He tears the arm together with the crown” (cf. Rashi on Numbers 32:17).