I heard the Messenger of Allah ﷺ say, "When you are three, two should not converse privately to the exclusion of the third."
حدثنا وكيع عن سفيان وعبد الرزاق قال حدثنا سفيان عن ابن دينار
سمعت رسول الله ﷺ يقول إذا كنتم ثلاثة فلا ينتجي اثنان دون واحد
Its isnad is sahih, and it is an abridgment of 5046. In one copy, the margin reads, "then there is no need."