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
In one narration the words are: I saw Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) talking privately with a man, and a man came in between the two of them. Then the narrator mentioned the narration as before.
Ahmad recorded it; its chain contains Abdullah bin Sa'id al-Maqburi, who is matruk (abandoned).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 12948