Abu Hurairah (May Allah be pleased with him) reported: Messenger of Allah (PBUH) said, "A believer owes another believer five rights: responding to greetings, visiting him in illness, following his funeral, accepting his invitation, and saying `Yarhamuk-Allah (May Allah have mercy on you),' when he says `Al-hamdu lillah (Praise be to Allah)' after sneezing". [Al-Bukhari and Muslim].