Narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When there are three people, two of them should not converse secretly excluding the third."
Hadith Referenceالادب المفرد / كتاب / 1168
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : أخرجه البخاري ، كتاب الإستئذان : 6288 و مسلم : 2183»