(15) باب تَحْرِيمِ مُنَاجَاةِ الاِثْنَيْنِ دُونَ الثَّالِثِ بِغَيْرِ رِضَاهُ
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.
Reference: Sahih Muslim 2183 a
In-book reference: Book 39, Hadith 48
USC-MSA web (English) reference(deprecated numbering scheme): Book 26, Hadith 5419