The Messenger of Allah ﷺ said, "When you are three, two should not converse privately to the exclusion of the third, for that grieves him."
حدثنا إسحاق حدثنا الأعمش عن أبي وائل
قال رسول الله ﷺ إذا كنتم ثلاثة فلا يتناجى اثنان دون صاحبهما فإن ذلك يحزنه
Its isnad is sahih. It was also narrated by Bukhari and Muslim, Tirmidhi, and Ibn Majah, as mentioned in Jami Saghir 842. In H: "falā yatanājiyān" (they shall not converse privately), and this was confirmed as correct from K.