Sahih Muslim Book 26, Hadith Number 5421.

Chapter : It is prohibited for two persons to converse secretly without the consent of the third one.

‘Abdullah reported Allah’s Messenger (may peace be upon him) 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.