Musnad Ahmad 27:117

Sufyan narrated to us, from Abdullah ibn Dinar, from Ibn Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "If you are three, then two of you must not whisper to the exclusion of the third." And he said another time: "The Prophet (peace and blessings of Allah be upon him) forbade that two men whisper to the exclusion of the third, if they are three."