“The
Muslim
has four things due from the Muslim: He should answer [by
saying
Yarhamuk-Allah (may Allah have mercy on you)] to him if he
sneezes
(and says Al-Hamdulillah); he should accept his invitation if
he
invites him; he should attend his funeral if he dies; and he should
visit him if he falls sick.”
Sunan Ibn Majah Book 6, Hadith 2