Giving salaam is Sunnah when joining a gathering and when leaving it
Question

                Is there any hadeethth which shows that we have to give salaam when we part from our gathering? We know that we have to give salaam when we meet, but when we part do we have to give salaam too? Is there any hadeeth with regards to this?.
Praise be to Allah.
      If a man joins a gathering it is Sunnah for him to give 
      salaams to people in this gathering, and when he wants to get up and 
      leave, he should give them salaam once again before he leaves. That is 
      because of the report narrated by Abu Dawood (5208) and al-Tirmidhi 
      (2706), and classed as hasan by al-Tirmidhi; and also narrated by Ahmad 
      (7793) from Abu Hurayrah (may Allah be pleased with him) who said: The 
      Messenger of Allah (blessings and peace of Allah be upon him) said: “When 
      one of you comes to a gathering, let him say salaam, and when he wants to 
      leave let him say salaam, for the former is not more of a duty than the 
      latter.” Classed as saheeh by al-Albaani in Saheeh Abi Dawood and 
      elsewhere. 

    Al-Mubaarakfoori (may Allah have mercy on him) said: 

    Al-Teebi said: i.e., just as the first salaam was telling 
    them that they are safe from his evil when he was present, the second salaam 
    is telling them that they are safe from his evil when he is absent. Giving 
    salaam when joining the gathering is not more important than giving salaam 
    when leaving the gathering; rather the second is more important. End quote. 

    Tuhfat al-Ahwadhi (2/402-403) 

    Al-Nawawi (may Allah have mercy on him) said: 

    The apparent meaning of this hadeeth is that it is obligatory 
    for the group to return the salaam of the one who gave them salaam and left 
    them. Imam al-Qaadi Husayn and his companion Abu Sa‘d al-Mutawalli said: The 
    custom among some people is to say salaam when leaving a gathering, and that 
    is a du‘aa’ (supplication) to which it is mustahabb to respond but is not 
    obligatory, because the greeting is only given when meeting and not when 
    parting. This is what they (these two scholars) said, but it was criticised 
    by Imam Abu Bakr al-Shaashi, the last of our companions, who said: This is 
    wrong, because giving salaam is Sunnah when leaving just as it is a Sunnah 
    when joining the gathering. This hadeeth proves that. And what al-Shaashi 
    said is the correct view. End quote. 

    Al-Adhkaar, p. 258 

    Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

    In this hadeeth we see that when a man enters upon a 
    gathering, he should say salaam. And when he wants to leave and he gets up 
    to leave the gathering, he should also say salaam, because the Prophet 
    (blessings and peace of Allah be upon him) enjoined that and said: “for the 
    former is not more of a duty than the latter.”, i.e., just as when you say 
    salaam when you enter, you should also say salaam when you leave. Hence when 
    a person enters the mosque, he sends salaams upon the Prophet (blessings and 
    peace of Allah be upon him), and when he leaves he also sends blessings upon 
    him. And when he enters Makkah for ‘Umrah or Hajj, he should start with 
    tawaaf, and when he leaves Makkah he should end with tawaaf, because tawaaf 
    is the greeting for Makkah for the one who enters it for Hajj or ‘Umrah, and 
    it is bidding farewell to Makkah for the one who has done Hajj or ‘Umrah and 
    is now leaving. This is a sign of the perfection of sharee‘ah, because it 
    makes the beginning and the end the same in such matters as these. End 
    quote. 

    Sharh Riyaadh al-Saaliheen, p. 
    990 

    And Allah knows best.