(52) باب النَّهْىِ عَنْ سَبِّ الأَمْوَاتِ،
It was narrated from Abu Hurairah that the Messenger of Allah said:
"The believer owes six duties toward his fellow believer: To visit him when he is sick, to attend his funeral when he dies, to accept his invitation, to greet him with Salam when he meets him, to reply to him (say: Yarhamuk Allah, may Allah have mercy on you) when he sneezes and to be sincere to him, whether he is absent or present." (Hasan).
Reference: Sunan an-Nasa'i 1938
In-book reference: Book 21, Hadith 122
English translation: Vol. 3, Book 21, Hadith 1940