"When you are three, two should not converse (privately) to the exclusion of their companion, because that makes him sad."  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا كُنْتُمْ ثَلاَثَةً فَلاَ يَتَنَاجَى اثْنَانِ دُونَ صَاحِبِهِمَا فَإِنَّ ذَلِكَ يُحْزِنُهُ  

"The Messenger of Allah ﷺ forbade two to converse (privately) to the exclusion of a third."  

نَهَى رَسُولُ اللَّهِ ﷺ أَنْ يَتَنَاجَى اثْنَانِ دُونَ الثَّالِثِ