It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Al-Salam (Peace) is one of the names of Allah which He has placed on the earth, so spread it among yourselves. For when a man greets a group with salam and they return his greeting, he has a degree of superiority over them because he reminded them (of Allah); and if they do not return his greeting, one who is better and purer than them (an angel) returns it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10353