John Gill Commentary 1 Samuel 2:34

"And this shall be the sign unto thee, that shall come upon thy two sons, on Hophni and Phinehas: in one day they shall die both of them." — 1 Samuel 2:34 (ASV)

This shall be a sign to you
A confirming one, that all which had been now said would be fulfilled:

that shall come upon your two sons, Hophni and Phinehas ;
which Eli would live to see fulfilled on them; and when it was, he might be assured the rest would be most certainly accomplished, and it was this:

in one day they shall die both of them ;
as they did in battle with the Philistines, (1 Samuel 4:11) .