I saw Ibn Umar speaking privately with a man when another man came and stepped between them. Ibn Umar struck his chest and said, "The Messenger of Allah ﷺ said, 'When two people are conversing privately, a third must not come between them without their permission.'"
رأيت ابن عمر يناجي رجلا، فدخل رجل بينهما، فضرب صدره وقال: قال رسول الله ﷺ: إذا تناجى اثنان فلا يدخل بينهما الثالث إلا بإذنهما.
Imam Ahmad narrated it (6225) from Nuh bin Maymun, and also (5949) from Surayj bin Numan, both of them from Abdullah bin Umar Umari, from Saeed Maqburi, who said, then he mentioned it.Abdullah bin Umar here is Ibn Hafs bin Asim bin Umar bin Khattab Umari, whom most scholars judged daif. Ahmad bin Hanbal said, "He is sound, there is no harm in him." Ibn Main said in one report, "There is no harm in him, his hadith may be written." Yaqub bin Shayba said, "He is trustworthy and truthful, though there is some inconsistency in his hadith." Ibn Adi said, "There is no harm in his narration, he is truthful."He was corroborated. His brother, also named Abdullah bin Umar Umari (the diminutive form, who is trustworthy), narrated it as well, from Saeed bin Abu Saeed Maqburi, from Ibn Umar, with similar wording.Daraqutni narrated it in al-Ilal (13/177) through the route of Abu Usama Hammad bin Usama, from Ubaydullah bin Umar, with this wording, as a raised (marfu) report. But Yahya bin Saeed Qattan and Abdullah bin Numayr differed with him, narrating it from Ubaydullah bin Umar as a stopped (mawquf) report. The mawquf version is stronger and more sahih in terms of its chain, but Abu Usama Hammad bin Usama is trustworthy and reliable, so his addition raising it to marfu is acceptable, especially since he was corroborated, as noted above.The hadith also has another chain that strengthens the marfu version. This is what Abu Naeem narrated in Hilyat Awliya (8/198), from Zayd bin Ali bin Abu Bilal Maqri, who said, Ali bin Bishr bin Salama narrated to us, who said, Ibrahim bin Yusuf Misri narrated to us, who said, Imran bin Uyayna narrated to us, from Abdul-Aziz bin Abu Rawwad, from Nafi, from Ibn Umar, who said, the Messenger of Allah ﷺ said, "A man should not sit with two men except with their permission, if the two of them are speaking privately together."He said this report is isolated, coming through Abdul-Aziz and Imran, the brother of Sufyan, with Ibrahim bin Yusuf being its sole transmitter, as noted by Abu Hasan, the hadith master, Daraqutni.Through these combined chains, the hadith rises to the level of hasan.