It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) forbade that when you are three people, two of you should not whisper to each other leaving the third one out.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4874
Hadith Gradingحکم دارالسلام:صحيح، خ: 6288، م: 2183، وهذا إسند ضعيف، محمد بن إسحاق مدلس وقد عنعن .