Sa'id al-Maqburi states: I came and sat with Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) while a man was talking with him. I came and sat between the two of them, so he struck my chest with his hand and said: Do you not know that the Prophet (peace and blessings be upon him) said: "When two people are talking privately, do not sit with them until you have asked their permission."
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 12947