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 whisper to each other excluding the third." The narrator says that I asked Sayyiduna Ibn 'Umar (may Allah be pleased with them both): What is the ruling if there are four? He replied: There is no harm in that.