Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with the same chain (from Abdullah bin Dinar, may Allah have mercy on him), it is narrated that Khalid bin Uqbah’s house, which is near the market, I and Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) were present there when a man came and wanted to whisper (speak in secret) to them (Ibn Umar, may Allah be pleased with them both), and there was no one with Abdullah bin Umar (may Allah be pleased with them both) except me and the man who wanted to speak in secret to him. So Abdullah called another man until we became four, then he said to me and the man who had been called, “You two move back a little, because I heard the Messenger of Allah (peace and blessings be upon him) say that (when there are three people) two should not whisper to each other excluding the third.”