Sahih al-Bukhari, Volume 8, Book 74, Hadith 305 (USC-MSA) ⓘ
Narrated `Abdullah: The Prophet said, "When you are three persons sitting together, then no two of you should hold secret counsel excluding the third person until you are with some other people too, for that would grieve him."
حَدَّثَنَا عُثْمَانُ، حَدَّثَنَا جَرِيرٌ، عَنْ مَنْصُورٍ، عَنْ أَبِي وَائِلٍ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ قَالَ النَّبِيُّ صلى الله عليه وسلم {إِذَا كُنْتُمْ ثَلاَثَةً فَلاَ يَتَنَاجَى رَجُلاَنِ دُونَ الآخَرِ، حَتَّى تَخْتَلِطُوا بِالنَّاسِ، أَجْلَ أَنْ يُحْزِنَهُ}.
also referenced under: Book 79, Hadith 62 (In-Book) & Hadith 6290 (DarusSalam)