Sahih Muslim Book 26, Hadith Number 5378.

Chapter : The right of a Muslim upon another Muslim is that his greeting should be responded.

Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: Five are the rights of a Muslim over his brother: responding to salutation, saying Yarhamuk Allah when anybody sneezes and says al-Hamdulillah, visiting the sick following the bier.’ Abd al-Razzaq said that this hadith has been transmitted as mursal hadith from Zuhri and he then substantiated it on the authority of Ibn Musayyib.