Salim al-Aftas narrates: When Sayyiduna Hamzah bin Abd al-Muttalib (may Allah be pleased with him), Sayyiduna Mus'ab bin Umayr (may Allah be pleased with him) and Sayyiduna Abdullah bin Jahsh (may Allah be pleased with him) were martyred, and they saw the good and the provision (that is, the reward), their desire for martyrdom increased, and they wished that their companions would also come to know of the good and provision they had been granted. So Allah Almighty said: "I will convey this to them on your behalf." Then Allah Almighty revealed this verse: "Do not ever think of those who are killed in the path of Allah (as dead)."
Al-Tabarani recorded it, and it is munqati' (broken) in its chain.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجهاد / 9543