كتاب السلام
39  
The Book of Greetings
(15)
Chapter: The Prohibition Of Two People Conversing Privately To The Exclusion Of A Third Without His Consent
(15)
باب تَحْرِيمِ مُنَاجَاةِ الاِثْنَيْنِ دُونَ الثَّالِثِ بِغَيْرِ رِضَاهُ ‏‏
Sahih Muslim 2183 a

Ibn Umar reported Allah's Messenger (ﷺ) as saying:

When there are three (persons), two should not converse secretly between themselves to the exclusion of the (third) one.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا كَانَ ثَلاَثَةٌ فَلاَ يَتَنَاجَى اثْنَانِ دُونَ وَاحِدٍ ‏"‏ ‏.‏
Reference : Sahih Muslim 2183a
In-book reference : Book 39, Hadith 48
USC-MSA web (English) reference : Book 26, Hadith 5419
  (deprecated numbering scheme)