Musnad Ahmad 27:1052

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Abdullah ibn Dinar: I was with Ibn ‘Umar, I and another man. Then a man came, and Ibn ‘Umar said: "Move back, for the Messenger of Allah (peace and blessings of Allah be upon him) said: 'When there are three, two must not whisper to each other excluding the one.'"