Is it permissible to greet inside the mosque or not? Is it not a sin?

Today I’m in with some friends and we are talking about some Islamic principals, and there one of my friend tell me that greeting with every one ( slaam,handshaking ) insides the mosque is not permissible,I’m wondered for a while then I asked him that tell me the reference of this hadith or verse of Quran but he can’t. So I decided to takes help of your peoples please guide me for this daily routine matter. Thanks