Prohibition of two Holding Secret Counsel to the Exclusion of Conversing together a Third Riyad as-Salihin 1599
Arabic hadith text
وعن ابن مسعود رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: "إذا كنتم ثلاثة، فلا يتناجى اثنان دون الآخر حتى تختلطوا بالناس، من أجل أن ذلك يحزنه" ((متفق عليه)).
Translation
Ibn Mas'ud (May Allah be pleased with him) said
The Messenger of Allah (ﷺ) said, "When three of you are together, two of you must not converse privately ignoring the third till the number increases, lest the third should be grieved."[Al-Bukhari and Muslim].