It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) said: A Muslim has five rights over another Muslim: to return the greeting of Salam, to accept an invitation, to participate in the funeral, to visit the sick, and to respond to a sneeze when the one who sneezes says "Alhamdulillah."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 8397