It is narrated from Sayyiduna Abu Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "A Muslim has four rights over a Muslim: that he accept when he invites him, visit him when he is ill, respond to him when he sneezes (and praises Allah), and accompany his funeral when he dies." This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.