Riyad as-Salihin 238
Abu Hurairah (May Allah bepleased with him) reported:
Messenger of Allah (ﷺ) said, "A believer owes another believer five rights: responding to greetings, visiting him in illness, following his funeral, accepting his invitation, and saying 'Yarhamuk-Allah (May Allah have mercy on you),' when he says 'Al-hamdu lillah (Praise be to Allah)' after sneezing". [Al-Bukhari and Muslim] . Muslim's narration is, "There are six rights of a Muslim upon a Muslim: When you meet him, greet him; when he invites you, respond to him; when he seeks counsel, give him advice; when he sneezes and praises Allah, say to him: 'May Allah have mercy on you (Yarhamuk-Allah)'; when he is sick, visit him; and when he dies, follow his funeral".
وعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال‏:‏ ‏"‏حق المسلم على المسلم خمس‏:‏ رد السلام، وعيادة المريض، واتباع الجنائز وإجابة الدعوة، وتشميت العاطس‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏ ‏.‏ 

و في رواية لمسلم: "حق المسلم على المسلم ست: اذا لقيته فسلم عليه، و اذا دعاك فأجبه، و اذا استنصحك فانصح له، و اذا عطس فحمد الله فشمته، و اذا مرض فعده، و اذا مات فاتبعه."