Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "A Muslim has six rights over a Muslim: that he greets him with salam when he meets him, accepts his invitation when he invites him, says 'Yarhamuk Allah' (may Allah have mercy on you) to him when he sneezes, attends his funeral when he dies, wishes him well even when he is absent, and loves for him what he loves for himself."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9713