"When you are three, two should not converse (privately) to the exclusion of their companion, because that makes him sad." (Using translation from Ibn Mājah 3775)  

قَالَ رَسُولُ اللَّهِ ﷺ «إِذَا كُنْتُمْ ثَلَاثَةً فَلَا يَتَنَاجَى اثْنَانِ دُونَ صَاحِبِهِمَا فَإِنَّ ذَلِكَ يُحْزِنُهُ»