Musannaf Ibn Abi Shayba 27:572

Waki’ narrated to us, from Sufyan, from Salim, from Sa’id ibn Jubayr, who said: When Hamza ibn ‘Abd al-Muttalib and Mus’ab ibn ‘Umayr were struck down on the day of Uhud, and they saw of the good what they saw, they said: "Would that our brothers knew what we have attained of good, so that they might increase in desire." Then Allah said: "I will convey [the message] on your behalf." So Allah revealed: "And do not think of those who have been killed in the way of Allah as dead; rather, they are alive with their Lord, receiving provision," up to His saying: "And that Allah does not waste the reward of the believers."