Hadith: When one of you arrives at a gathering, he should say the greeting of Salām. He should also do so when he wants to depart, for the first greeting is not more meritorious than the last

عَنْ أَبِي هُرَيْرَةَ، قَالَ: قَالَ رَسُولُ اللَّهِ -صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ-: «إذا انْتَهى أَحَدُكُم إلى المجْلِسِ فَلْيُسَلِّم، فإذا أرادَ أن يقومَ فَلْيُسَلِّمْ، فَلَيْسَت الأُولَى بأحَقَّ مِن الآخِرَةِ».
[صحيح] - [رواه أبو داود والترمذي والنسائي في الكبرى وأحمد]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "When one of you arrives at a gathering, he should say the greeting of Salām. He should also do so when he wants to depart, for the first greeting is not more meritorious than the last."
Sahih/Authentic. - [An-Nasaa’i]

Explanation

This Hadīth shows one of the etiquettes of greeting. When one enters a place where others are sitting, one should greet them with the Salām greeting. The same should be done when leaving the session, as the Prophet (may Allah's peace and blessings be upon him) said: The first time (of greeting) is not more meritorious than the second. This means that just as a person greets with the Salām when he comes in, he should do so when he leaves. Similarly, when one enters a mosque, one should send peace and blessings upon the Prophet (may Allah's peace and blessings be upon him) and so should one do when leaving the mosque. The first greeting conveys to the congregation that they will be safe from any possible harm on the part of the person who is coming in. Likewise, the second greeting tells them that they will be safe from his evil when he goes away from them. This is an aspect of the perfection of the Shariah: It applies the same ruling of greeting whether the person is coming or leaving. Indeed, the Shariah is from the All-Wise, the All-Knowing.