Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Five things are the right of a Muslim over a Muslim: returning the greeting of salam, responding to one who sneezes, visiting the sick, attending the funeral, and accepting an invitation."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5847