Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When you are three, then two of you should not converse secretly excluding the third until you join other people, for this manner will grieve him (the third person)." Agreed upon.
Hadith Referenceمشكوة المصابيح / كتاب الآداب / 4965
Hadith Gradingالألبانی:مُتَّفَقٌ عَلَيْهِ | زبیر علی زئی:متفق عليه
Hadith Takhrij´تحقيق و تخريج: محدث العصر حافظ زبير على زئي رحمه الله` «متفق عليه، رواه البخاري (6290) و مسلم (2184/37)»