Tirmidhi

Hadith Commentary:
(1)
This hadith, from the perspective of its implied meaning (mafhum mukhalif), corresponds with the chapter heading; that is, if there are more than three people, then two of them may converse privately. Accordingly, in one narration, the transmitter of the hadith, Abu Salih, asked Ibn Umar radi Allahu anhuma, “What if there are four individuals?” He replied:
There is no harm in that.
(Sunan Abi Dawud, Al-Adab, Hadith: 4852) (2)
It was the practice of Ibn Umar radi Allahu anhuma that if he wished to speak confidentially with someone in the presence of three people, he would include a fourth person.
In any case, for two people to whisper to each other, excluding the third, or to speak in a language that he does not understand, is extremely distressing for him and is also contrary to his honor and dignity.
And Allah knows best.

Hadith Commentary: Benefits and Issues: When three individuals mingle with other people, then in the case of two people whispering, the third person will be able to engage in conversation with the other people. In this way, he will not experience any distress.

Takhrij:
[بخاري :2688],
[مسلم، السلام 5696],
[بلوغ المرام : 1237]
[تحفة الأشراف 56/7]

Vocabulary:
«يحزنه» with a kasrah on the letter zaa, like «أكرم يكرم», and with a dammah on the zaa, like «نصر ينصر», both are transitive and mean "to make someone sad." «ولا يحزنك الذين يسارعون فى الكفر» "Let not those who hasten into disbelief grieve you" [آل عمران : 19]. However, with a fathah on the zaa, «سمع» is intransitive and means "to be sad." «لَا تَحْزَنْ إِنَّ اللَّـهَ مَعَنَا» "Do not grieve; indeed, Allah is with us." [9-التوبة:40]

Benefits:

➊ The reason why a third companion becomes sad when two people whisper to each other is that he may think they are plotting against him, or at the very least, he will feel that they did not consider him worthy of being included in their secret.

➋ After joining other people, two individuals may whisper to each other, even if another person joins them. Because when two people whisper to each other, the third is not alone; rather, there is another person with him, so they can converse among themselves. Malik narrated from Abdullah bin Dinar that once he and Abdullah bin Umar radi Allahu anhuma were at the market near the house of Khalid bin Uqbah. A man came who wanted to speak privately with Abdullah bin Umar radi Allahu anhuma, and at that time, besides Abdullah bin Umar, there was no one else with him except me. So he called another man and said to me and that man: "You both wait a little, for I heard the Messenger of Allah sallallahu alayhi wa sallam say..." Then he narrated this hadith: "When you are three..." etc. [موطا]

➌ Imam Malik rahimahullah said: Two people cannot whisper to each other leaving out one companion, so likewise, more than two people cannot whisper to each other leaving out one companion. For example, if three or ten people separate one of their companions and whisper among themselves, this will cause even more grief than if only two people separated and whispered. Therefore, as long as there is not another person with him, it is not permissible for the other companions to whisper among themselves. [فتح الباري]

➍ If two people are speaking secretly and a third comes to listen, it is not permissible for him, nor does his coming make it forbidden for them to whisper. Sa'id Maqburi said: I passed by Abdullah bin Umar radi Allahu anhuma while a man was talking to him. I stood by them, so he pushed me in the chest and said: "When you see two people talking, do not stand or sit with them until you seek permission." [صحيح الأدب المفرد للبخاري 1166/889]
The Messenger of Allah sallallahu alayhi wa sallam said: "Whoever eavesdrops on the conversation of people who dislike that, molten lead will be poured into his ears on the Day of Resurrection." [عن ابن عباس احمد، ابوداود، الترمذي، صحيح الجامع 6370]

Takhrij:
«أخرجه البخاري، الاستئذان، باب إذا كانوا أكثر من ثلاثة...، حديث:6290، ومسلم، السلام، باب تحريم مناجاة الثنين دون الثالث بغير رضاه، حديث:2184.»©Explanation:
In this hadith, it is taught that it is forbidden to whisper or converse secretly while ignoring a companion.
From this, we learn to respect human emotions and feelings, and that one should not do anything that causes distress to another, or makes him feel that he is being regarded as an outsider rather than one of their own, or that he becomes suspicious and apprehensive that the other two are conspiring against him and trying to deceive him.
Such behavior hurts the feelings and emotions of the other person; therefore, in a gathering of three people, it is prohibited for two to separate themselves and whisper or converse secretly.