صَحِيحُ مُسْلِمٍ
Sahih Muslim 2183 a
The Book of Greetings
The Prohibition Of Two People Conversing Privately To The Exclusion Of A Third Without His Consent
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 .
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.
Sahih Muslim, 2183 aSahihصحيح