The Messenger of Allah ﷺ said, "When three people are together, two should not converse privately to the exclusion of the third."
حدثنا ابن نمير حدثنا عبيد الله عن نافع
أن رسول الله ﷺ قال إذا كانوا ثلاثة فلا يتناجى اثنان دون واحد
Its chain of transmission is sahih, and it is a repetition of 6264.