Sunan an-Nasa'i 1938
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).
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا مُحَمَّدُ بْنُ مُوسَى، عَنْ سَعِيدِ بْنِ أَبِي سَعِيدٍ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏
"‏ لِلْمُؤْمِنِ عَلَى الْمُؤْمِنِ سِتُّ خِصَالٍ يَعُودُهُ إِذَا مَرِضَ وَيَشْهَدُهُ إِذَا مَاتَ وَيُجِيبُهُ إِذَا دَعَاهُ وَيُسَلِّمُ عَلَيْهِ إِذَا لَقِيَهُ وَيُشَمِّتُهُ إِذَا عَطَسَ وَيَنْصَحُ لَهُ إِذَا غَابَ أَوْ شَهِدَ ‏"‏ ‏.‏