Sayyiduna Dahr al-Aslami (may Allah be pleased with him) narrates that he heard the Prophet (peace and blessings be upon him), during the journey to Khaybar, say this to Sayyiduna Amir bin al-Akwa' (may Allah be pleased with him). After that the narrator mentioned the whole hadith, in which, further on, are these words: The Prophet (peace and blessings be upon him) said: "May Allah Almighty have mercy on him." Sayyiduna Umar (may Allah be pleased with him) said: "O Messenger of Allah, by Allah, (martyrdom) has become certain for him. If only you had let us benefit from him longer (that is, had you prayed for his life), it would have been fitting." The narrator says: He was martyred on the occasion of the Battle of Khaybar.
This narration has already preceded in the chapter on the Battle of Khaybar. Al-Tabarani recorded it, and its narrators are thiqat (trustworthy).