Sayyiduna Abdullah bin Umayr (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) was returning from Uhud, he passed by Mus'ab bin Umayr (may Allah be pleased with him) and his fellow martyrs, stopped by them and said: "I bear witness that you are alive with Allah. So visit them and pray for them. By the One in whose Hand is the soul of Muhammad (peace and blessings be upon him), no one greets them with salam but they return his greeting, until the Day of Resurrection."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17975