Two should not converse to the exclusion of a third
بَابُ لا يَتَنَاجَى اثْنَانِ دُونَ الثَّالِثِ
Al-Adab Al-Mufrad, Book of Behaviour with people(view book)
Al-Adab Al-Mufrad 1168
'Abdullah reported that the Messenger of Allah ﷺ said, "When there are three people, two should not converse together to the exclusion of the third."
حَدَّثَنَا إِسْمَاعِيلُ قَالَ: حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللهِ، أَنَّ رَسُولَ اللهِ ﷺ قَالَ: إِذَا كَانُوا ثَلاَثَةً، فَلاَ يَتَنَاجَى اثْنَانِ دُونَ الثَّالِثِ.
Classification
Sahih (Authentic) [Al-Albani]