In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Shaikh Saaleh Al-Fawzaan [may Allaah preserve him] says:
And one of those etiquettes [to be observed in a] social gathering [or when sitting together] is that two [people] should not have a private conversation in isolation of a third person [i.e. a third person who is also present in the same gathering or sitting], because that will offend him. He may harbour an evil suspicion towards them – that they are either conspiring against him or reviling him, or backbiting him, or may think that they are looking down on him. The Messenger [sallal-laahu-alayhi-wasallam] said, ‘’When three persons are together, then no two of them should hold secret counsel excluding the third person’’.
The Messenger [sallal-laahu-alayhi-wasallam] said, ‘’When you are three persons sitting together, then no two of you should hold a secret counsel excluding the third person until you are with some other people too, for that would grieve him’’. [End of quote. Ref 1]
This hadeeth is [about the] etiquettes that should be observed when sitting together. If there are three people [together], it is forbidden for two of them to speak in private; because if they speak in private without the third person, he will harbour some doubts – fearing that they are talking about him. Also if they speak in secret without him, he will feel that they are looking down on him and that they do not give him any value, and that is why they hide their affair from him and speak in private without him, because they do not trust him. This will enter his heart and that is why the Prophet [sallal-laahu-alayhi-wasallam] said, ‘’For that would grieve him’’- Meaning: It will bring about sadness in his heart, so he says [to himself], ‘’They are either speaking about me or looking down on me’’, so he becomes sad. Therefore, from the etiquettes of sitting together is that one speaks openly, and speaking should not be between two people in isolation of a third person. As for when there are many people in a sitting – more than three – there is no harm if two people speak in private because the other people are many and they will not harbour anything in their hearts.
This is from the etiquettes of sitting together. The hadeeth forbids two people from speaking to each other in private, whilst isolating a third [person who is also present in the same sitting]. It shows that if there are more than three people, then there is no harm if two people speak in private due to the statement of the Prophet [sallal-laahu-alayhi-wasallam], ‘’Until you are with some other people too.’’ So if the reason behind the caution is non-existent [i.e. if there are many other people in the same sitting], then there is no harm [if two people speak in private]. [End of quote. Ref 2]
Imaam Abdul Azeez Bin Baaz [rahimahullaah] said:
And also if there are four people, they [i.e. three people] should not speak privately in isolation of the fourth person. The intent behind this is that there should be more than one person remaining if the rest are having a private conversation. If a group speaks privately in isolation of another group, then there is no harm in doing so as has preceded in the hadeeth reported by Aa’isha that the Prophet spoke to Faatimah in private [about something] in the house, but not to his wives. [End of quote. Ref 3] Here is the Hadeeth: Imaam Bukhaari Said, ‘’Chapter: Whoever has a confidential talk with somebody in front of the people and the latter does not disclose his companion’s secret, but when his companion dies, he discloses it. http://www.salaficentre.com/2016/10/heart-warming-incident-prophet-sallal-laahu-alayhi-wasallam-beloved-daughter-faatimah-radiyallaahu-anhaa/ [Ref 4]
It Is Not Allowed to Listen to The Conversations of The People If They Are Not Pleased With That – By Shaikh Saaleh Al-Fawzaan
It is forbidden for a person to just come and sit with a person who is talking to another person because it may be that they are engaged in a private conversation. The Prophet [sallal-laahu-alayhi-wasallam] said, “Whoever listens to the talk of some people who do not like him [to listen] or they run away from him, then molten lead will be poured into his ears on the Day of Resurrection. [Sahih Al-Bukhaari Number 7042]. This is a severe threat, so one should not listen to the speech of the people if they do not want anyone to know what they are talking about. [End of quote. Ref 5]
It Is Not Allowed to Listen to The Conversations of The People If They Are Not Pleased With That – By Shaikh Zayd Bin Haadi
It may be that two or more people gather to speak about a particular affair that concerns them, and they do not want anyone else to know about it; but then a person or more than one person becomes an obstacle by listening to their speech and [wants] to know what they are talking about, whilst they are not pleased with that. This is an act of transgression and foolishness – that a person involves himself into something that does not concern him. And from the perfection of a person’s Islaam is to leave that which does not concern him. However, there is an exception to this prohibition such as when a person listens into the speech of those who are focused or [involved] in seeking to breach the state of safety and security of the Muslims. [Ref 6]
[Ref 1: An Excerpt from ‘It’haaf At-tullaab Bi-Sharh Mandhoomah Al-Aadaab’ page 149. paraphrased]
[Ref 2: An Excerpt from ‘Tasheelul Ilhaami Bi-Fiqhil Ahaadeethi Min Bulooghil Maraam’ 6/172 paraphrased]
[Ref 3: An Excerpt from ‘Al-Hulalul Ibreeziyyah Min At-taleeqaat Al-Baaziyyah Alaa Saheeh Al-Bukhaari’ Vol 4′ page 192′ footnote number 2′. paraphrased]
[Ref 4: Al-Hulalul Ibreeziyyah Min At-taleeqaat Al-Baaziyyah Alaa Saheeh al-Bukhaari Vol 4′ page 191]
[Ref 5: An Excerpt from ‘It’haaf At-Tullaab Bi-Sharhi Mandhoomah Al-Aadaab’ page 149′ paraphrased]
[Ref 6: An Excerpt from ‘Awnul Ahadis Samad Sharh Al-Adabil Mufrad’ Vol 3 page 281′. paraphrased]