Sahih al-Bukhari 6053
In-book no 6053حَدَّثَنَا 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}.
Narrated `Abdullah: The Prophet (ﷺ) 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."