It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that the Prophet (peace and blessings of Allah be upon him) said: "It is not lawful for a man to separate two people sitting together by sitting between them, except with their permission."
Hadith Referenceالادب المفرد / كتاب المجالس / 1142
Hadith Gradingالألبانی:حسن
Hadith Takhrij«حسن : أخرجه أبوداؤد ، كتاب الأدب : 4845 و الترمذي : 2752»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: If two people are sitting together, it is against etiquette to intrude and sit between them. It is possible that they are discussing something confidential or wish to sit together.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 1142