The Muslim's right over his fellow Muslim is six." It was said: What are they, O Messenger of Allah? He said: "When you meet him, greet him with peace; when he invites you, accept his invitation; when he seeks your advice, advise him; when he sneezes and praises Allah, say Tasmīt to him; when he falls sick, visit him; and when he dies, follow him

Ash-Shawkāni said: What is meant by (The Muslim's right) is that it should not be neglected, and fulfilling it is either obligatory or firmly recommended in such a way that makes it similar to the obligation that must not be neglected.
Responding to a greeting of peace is an individual obligation if the greeting is directed at a single person, and if directed at a group, it suffices if one of them responds. As for initiating the greeting of peace, it is basically an act of Sunnah.
One of the Muslim’s rights over his fellow Muslims is to visit him when he is sick, as it brings joy and comfort to his heart and is considered a communal obligation.
It is obligatory to respond to an invitation unless it involves a sin. If the invitation is to a wedding banquet, the majority hold that it is obligatory to respond unless there is a valid excuse under the Shariah. However, if the invitation is for something other than a wedding banquet, the majority consider it recommended.
Saying Tashmīt to the sneezer is obligatory upon whoever hears him praising Allah.
The perfection of the Shariah and its emphasis on strengthening the bonds of society and faith, as well as the ties of love among its members.
(Say Tasmīt to him) and in some versions "Tashmīt" with "s" and with "sh": a supplication for goodness and blessing. It was said that "Tashmīt" means: May Allah keep you away from gloating and protect you from what may cause your enemy to rejoice over you. "Tasmīt" means: may Allah guide you to the straight path.