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: "There are no two Muslims except that there is a veil from Allah between them. So when one of them speaks foully to his companion, he tears his veil; and when he says: 'O disbeliever!', then one of the two has certainly become a disbeliever (in ruling)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10498