The Book of Greetings
39/15. Chapter:
The Prohibition Of Two People Conversing Privately To The Exclusion Of A Third Without His Consent
Sahih Muslim
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، وَأَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ وَابْنُ نُمَيْرٍ وَأَبُو كُرَيْبٍ - وَاللَّفْظُ لِيَحْيَى - قَالَ يَحْيَى أَخْبَرَنَا وَقَالَ الآخَرُونَ، حَدَّثَنَا أَبُو مُعَاوِيَةَ، عَنِ الأَعْمَشِ، عَنْ شَقِيقٍ، عَنْ عَبْدِ اللَّهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا كُنْتُمْ ثَلاَثَةً فَلاَ يَتَنَاجَى اثْنَانِ دُونَ صَاحِبِهِمَا فَإِنَّ ذَلِكَ يُحْزِنُهُ " .
'Abdullah reported Allah's Messenger (ﷺ) as saying: If you are three, two should not converse secretly to the exclusion of your companion for that hurts his feelings.
Reference : Sahih Muslim 2184bIn-book reference : Book 39, Hadith 51USC-MSA web (English) reference : Book 26, Hadith 5422