It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to forbid that when there are three people, two of them should converse secretly excluding the third.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6057
Hadith Gradingحکم دارالسلام:إسناده صحيح ، خ: 6288، م: 2183.