1 Samuel 2:34

For the age of manhood is the time of administering the priesthood. Therefore the imitator of the reprobate teacher is brought to manhood when he is promoted to the height of holy orders. When indeed he has reached that age, he dies, because whoever has approached so great a ministry unworthily, or has lived unworthily in it, is condemned. Whence Paul also, making mention of the Lord's body and blood, says: "He who eats and drinks unworthily, eats and drinks judgment to himself" (1 Cor. 11:29). Therefore they die in manhood, because they perish from the presumption and boldness of a higher ministry. Hence therefore, hence indeed let the reckless take heed, and let them not eagerly seek but rather dread to undertake the burdens of so great a ministry. For he who is foretold to die when he reaches manhood lives until he reaches it, because indeed all who are weak and unequal to so great a ministry, if they consider the measure of their own smallness, have a place in the holy Church in which, guarding themselves, they may live. Whence the Lord also declares through Moses, saying: "If a beast touches the mountain, it shall be stoned" (Exod. 19:12; Heb. 12:20). Hence it is also that when Lot went out from Sodom, he refrained from ascending the mountain and turned aside to Zoar, a small city (Gen. 19:20ff.), lest while still small he strives for higher things and, striking against manhood, dies. For we leave Sodom when we turn away from the fires of burning desire. We do not immediately ascend to lofty things, because we do not touch the heights of a superior ministry, while we consider our own weakness by measuring it, lest if we despise keeping the measure of our weak age, we who have lived as little ones may die in manhood. As a sign indeed of the house about to die, both sons of Eli are foretold to die in one day. […]

This will be the sign for you, etc. These words pertain to Eli and Samuel, because the death of Eli's sons and Samuel's election as priest not from the lineage of Aaron signifies the death not of men, but of the old priesthood, and the …

37. We have shown above that in the two sons of Eli the twofold order of the old priesthood is designated. They certainly died on one day, because they came together simultaneously in the death of the Redeemer. Moreover, the death of the priests is fittingly recorded as placed as a sign of the house that was to die, because when the shepherds perish, it is necessary that the flock follow to the same destruction. And because they were deceived by a false reasoning, they are said to die in the day.

They shall die both of them - Hophni and Phinehas were both killed very shortly after in the great battle with the Philistines in which the Israelites were completely routed, and the ark taken. See 1Sa 4:1-11.