It is narrated from Sayyiduna Abu Mas'ud al-Badri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to say: "A Muslim has four rights over his (Muslim) brother: that he accept his invitation when he invites him, that he attend his funeral when he dies, that he visit him when he is sick, and that he respond to him when he sneezes."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14806