The Messenger of Allah ﷺ said, "A believer owes his fellow believer six duties. He visits him when he is sick, attends his funeral when he dies, accepts his invitation when invited, greets him with peace when he meets him, says 'May Allah have mercy on you' when he sneezes, and gives him sincere counsel whether he is present or absent."
أخبرنا قتيبة قال حدثنا محمد بن موسى عن سعيد بن أبي سعيد
أن رسول الله ﷺ قال للمؤمن على المؤمن ست خصال يعوده إذا مرض ويشهده إذا مات ويجيبه إذا دعاه ويسلم عليه إذا لقيه ويشمته إذا عطس وينصح له إذا غاب أو شهد
[Its isnad is hasan] Tirmidhi narrated it, Chapters on Manners, chapter on what has been said regarding responding to the sneezer, no. 2737, from Qutayba bin Saeed with this chain, and he said, hasan sahih, and it is also in Kubra, no. 2065. The hadith has corroborating narrations, see Majma al-Zawaid 8/185 and others. Muhammad bin Musa is Ibn Abu Abdullah Fitri, Abu Abdullah Madani, hasan al-hadith.