Sayyiduna Abu Umamah (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) sent Amr bin al-Tufayl to Khaybar so that he might seek reinforcements for the Prophet (peace and blessings be upon him) from his people. He said: "O Amr! Go and seek reinforcements for us from your people." Amr said: "O Messenger of Allah! You are sending me while the time of battle has drawn near?" The Prophet (peace and blessings be upon him) said: "Are you not pleased to become the messenger of the Messenger of Allah?"
Al-Tabarani recorded it; its chain contains Ali bin Yazid al-Alhani, who is da'if (weak).