Imam al-Zuhri (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said: "Five things are obligatory upon a Muslim toward his Muslim brother: returning the greeting of peace (salam), responding to one who sneezes, accepting an invitation, visiting the sick, and attending the funeral prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14530