Al-Adab Al-Mufrad, Book 47, Hadith 1169 (English) ⓘ
'Abdullah reported that the Prophet, may Allah bless him and grant him peace, said, "When there are three people, two should not converse together to the exclusion of the third for that would grieve him."
حَدَّثَنَا عُمَرُ بْنُ حَفْصٍ قَالَ: حَدَّثَنِي أَبِي، قَالَ: حَدَّثَنَا الأَعْمَشُ قَالَ: حَدَّثَنِي شَقِيقٌ، عَنْ عَبْدِ اللهِ قَالَ: قَالَ النَّبِيُّ صلى الله عليه وسلم: إِذَا كُنْتُمْ ثَلاَثَةً فَلاَ يَتَنَاجَى اثْنَانِ دُونَ الثَّالِثِ، فَإِنَّهُ يُحْزِنُهُ ذَلِكَ.
also referenced under: Hadith 1169 (Reference) & Book 47, Hadith 1169 (In-Book)