Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "When there are three people, two of them should not talk privately, leaving out their companion."
Al-Bazzar recorded it; its chain contains Abdullah bin Umar al-Umari, whom more than one scholar declared thiqah (trustworthy), though there is weakness in him. The rest of its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 12950