Thaabit ibne-dahhak (ral) anhu narrates that Nabi (sal) said: cursing a believer is like killing him.
A’isha Radiyallahu anha narrates that rasulullahi (sal) said: The man who is most hateful to Allah is the one who quarrels and argues the most.
Abdullah ibne-Abbas Rdiyallahu anhuma narrates that Nabi (sal) said: Do not quarrel with your brother, nor joke with him(in a manner which will hurt him), and do not make a promise to him which you do not honour