Sa'id bin Jubayr narrates: When Hamzah and his companions were martyred on the day of Uhud, they said: "If only those we left behind knew how much reward Allah has given us, so that it might be a source of reward for them too." So Allah, the Mighty and Majestic, said: "I will inform them." And Allah, the Blessed and Exalted, revealed this verse: "And never think of those who have been killed in the way of Allah as dead" [Al Imran: 169], to the end of the verse.
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2933