The Prophet ﷺ said, "When three people are together, two of them should not converse privately to the exclusion of the third, for that grieves him."
حدثنا يحيى عن الأعمش عن شقيق
عن النبي ﷺ إذا كانوا ثلاثة فلا يتناجى اثنان دون صاحبهما فإن ذلك يحزنه
Its chain of transmission is sahih, and it is a repetition of 4093.