Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: On returning from Uhud, the Prophet (peace and blessings be upon him) passed by Sayyiduna Mus'ab bin Umayr (may Allah be pleased with him), and he stopped by him together with his Companions and said: "I bear witness that you are alive in the presence of Allah Almighty." (Then he said to his Companions:) "Visit them and send greetings of peace upon them. By the One in whose hand is the soul of Muhammad (peace and blessings be upon him), until the Day of Resurrection, whoever sends greetings of peace upon them, they will reply to him."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains Abd al-A'la bin Abdullah bin Abi Farwah, who is matruk (abandoned).