The Messenger of Allah ﷺ said, "Two people must not converse privately to the exclusion of a third."
حدثنا عفان حدثنا عبد العزيز بن مسلم حدثنا عبد الله بن دينار
قال رسول الله ﷺ لا يتناجى اثنان دون واحد
Its chain of transmission is sahih, and it is an abridgment of 5281.