It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) said: "When you are three people, then two of you should not whisper to each other excluding the third." We asked: What if there are four? He said: "Then there is no harm."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4685
Hadith Gradingحکم دارالسلام:إسناده صحيح، خ : 6288، م : 2183