It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When you are three people, then two of you should not start whispering to each other, leaving the third one out."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5258
Hadith Gradingحکم دارالسلام:إسناده صحيح ، خ : 6288 ، م : 2183 .