The Messenger of Allah ﷺ said, "Two people should not converse privately to the exclusion of a third."
حدثنا يحيى عن عبيد الله عن نافع
أن رسول الله ﷺ قال لا يتسار اثنان دون الثالث
No references are available.
Its isnad is sahih. It is an abridgment of 4564.