The Prophet (peace and blessings be upon him) said:
"Two people should not converse privately, leaving out the third, because that grieves him (the third)."
Hadith Referenceصحیح الجامع الصغیر / حرف اللام ألف / 7799
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[د] عن ابن مسعود وابن عمر. الصحيحة ٢٤٠٢: ق: ابن مسعود١.