Abdullah reported Allah’s Messenger (SAW) as saying:
If you are three, two amongst you should not converse secretly between yourselves to the exclusion of the other (third one), unti some other people join him (and dispel his loneliness), for it may hurt his feelings.
Reference : Sahih Muslim 2184 a
In-book reference : Book 39, Hadith 50
USC-MSA web (English) reference : Book 26, Hadith 5421