Al-Mu'jam al-Awsat 3700 — On returning from the Battle of Uhud

Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: On returning from the Battle of Uhud, the Messenger of Allah (peace and blessings be upon him) passed by Sayyiduna Mus'ab bin Umayr (may Allah be pleased with him), and he stopped by him and his companions and said: "I bear witness that you are alive with Allah. So visit them and greet them with salam, for by the One in whose Hand is the soul of Muhammad (peace and blessings be upon him), until the Day of Resurrection, whoever greets them with salam, they will return his greeting."
Imam al-Tabarani (may Allah have mercy on him) said: "This hadith is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) only through this chain; Abu Bilal is alone in narrating it."