יְחִ֥י רְאוּבֵ֖ן וְאַל־יָמֹ֑ת וִיהִ֥י מְתָ֖יו מִסְפָּֽר׃ (ס)
Viva Rubén, y no muera; Y sean sus varones en número.
Chizkuni
יחי ראובן ואל ימות, “may Reuven live and not die;” Moses refers to the period when the members of that tribe would be in the vanguard of crossing the Jordan and battling the Canaanites.
Da'at Zekenim
יחי ראובן ואל ימות, “may Reuven live and not die.” When the tribe of Reuven will fulfill its promise and march as vanguard in front of the army conquering the land of Canaan, (together with the tribe of Gad and half the tribe of Menashe, as per Numbers 32,20-42) they will not suffer a single casualty during that campaign.