It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: "When you are three people, then two should not converse privately excluding the third, for this will distress him."
Hadith Referenceالادب المفرد / كتاب / 1169
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : أخرجه البخاري ، كتاب الاستئذان : 6290 و مسلم : 2184 و أبوداؤد : 4851 و الترمذي : 2825 و ابن ماجه : 3775»