Sahih Muslim , Book 26, Hadith 5419 (USC-MSA) ⓘ
Ibn Umar reported Allah's Messenger 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 " .
also referenced under: Book 39, Hadith 48 (In-Book) & Hadith 2183 (Reference)