Silsilat al-Ahadith al-Sahihah

Saeed Maqbari says: A man was talking to Sayyiduna Abdullah bin Umar (may Allah be pleased with them), and I also went and sat there. Ibn Umar (may Allah be pleased with them) struck his hand on my chest and said: "Do you not know that the Messenger of Allah (peace and blessings be upon him) said: 'When two people are having a private conversation, do not sit with them without their permission.'"