Sunan Ibn Majah, Volume 2, Book 6, Hadith 1434 (DarusSalam) ⓘ

It was narrated from Abu Mas’ud that the Prophet said: “The Muslim has four things due from the Muslim: He should answer [by saying Yarhamuk-Allah (may Allah have mercy on you)] to him if he sneezes (and says Al-Hamdulillah); he should accept his invitation if he invites him; he should attend his funeral if he dies; and he should visit him if he falls sick.”

حَدَّثَنَا أَبُو بِشْرٍ، بَكْرُ بْنُ خَلَفٍ وَمُحَمَّدُ بْنُ بَشَّارٍ قَالاَ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، حَدَّثَنَا عَبْدُ الْحَمِيدِ بْنُ جَعْفَرٍ، عَنْ أَبِيهِ، عَنْ حَكِيمِ بْنِ أَفْلَحَ، عَنْ أَبِي مَسْعُودٍ، عَنِ النَّبِيِّ ـ صلى الله عليه وسلم ـ قَالَ ‏ "‏ لِلْمُسْلِمِ عَلَى الْمُسْلِمِ أَرْبَعُ خِلاَلٍ يُشَمِّتُهُ إِذَا عَطَسَ وَيُجِيبُهُ إِذَا دَعَاهُ وَيَشْهَدُهُ إِذَا مَاتَ وَيَعُودُهُ إِذَا مَرِضَ ‏" ‏ ‏.‏