Riyad us-Salihin Hadith 238

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".