Musannaf Ibn Abi Shayba 36:217

Waki’ narrated to us, from Sufyan, from Salim, from Sa’id ibn Jubayr, regarding the verse: "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." He said: When Hamza ibn ‘Abd al-Muttalib and Mus’ab ibn ‘Umayr were struck down on the day of Uhud, they said, "We wish our brothers knew what good has befallen us, so that they might increase in desire [for martyrdom]." Allah said, "I will inform them on your behalf." Then was revealed: "And do not think of those who have been killed in the way of Allah as dead," up to His saying: "the doers of good."