Musnad Ahmad 27:1490

Surayj narrated to us, Abdullah narrated to us, from Sa’id al-Maqburi, who said: I sat with Ibn Umar, and there was a man with him speaking to him, so I entered with them. He struck my chest with his hand and said: "Do you not know that the Messenger of Allah, may Allah bless him and grant him peace, said: 'If two people are whispering to each other, do not sit with them until you ask their permission'?"