Musnad Ahmad 27:1765

Nuh narrated to us, Abdullah informed us, from Sa’id al-Maqburi, who said: I saw Ibn Umar whispering to a man, and a man entered between them, so he struck his chest and said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If two are whispering, let not a third enter between them except with their permission." Ya’qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: Sa’id ibn Abi Sa’id al-Maqburi narrated to me, from ‘Ubayd ibn Jurayj, the freed slave of Banu Taym, and he mentioned the hadith.