It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "When you are three people, two of you should not whisper to each other excluding the third."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5425
Hadith Gradingحکم دارالسلام:إسناده صحيح ، خ : 6288، م :2183.