Hadith 19 and 20:
The right of a muslim upon another muslim is that his greeting should be responded
Abu Huraira(raa) reported Allah’s Messenger may peace be upon him as saying :
“Six are the rights of a muslim over another muslim ” It was then said to him Allah’s Messenger (may peace be upon him) “What are these ?”
Thereupon he said:
“When you meet him offer him greetings, when he invites you to feast accept it, when he seeks your council give him,
and when he sneezes and says “Alhamdulillah” you say Yarhamuk Allah (may Allah show mercy on you),
and when he falls sick visit him, and when he dies follow his bier (janaazah)
Similarly there is one more Hadith
Abu Huraira(raa).reported Allah’s Messenger may peace be on him as saying:
“Five are the rights of a muslim over his brother: Responding to salutation, saying YarhamukAllah when somebody sneezes and says Alhamdulillah,
visiting the sick, following the bier(janaazah).”
Hadith no 5379 and 5378, Sahih Muslim, Kitab-as-salaam
PS: Some of you might have noticed that second hadith says 5 things but mentions only 4 things, i checked at quite a few places but found same hadith everywhere if any one knoes the correct one kindly let me know.
Allah knows best what is right and what is wrong