The Messenger of Allah ﷺ said, "When three people are together, two should not converse privately to the exclusion of the third."
أن رسول الله ﷺ قال: إذا كان ثلاثة، فلا يتناجى اثنان دون الثالث.
Malik related it in "Speech" (14) from Nafi, from Ibn Umar, and mentioned it. Bukhari related it in "Seeking Permission" (6288), and Muslim in "Greetings" (2183), both through the route of Malik with the same wording.Abu Dawud related it (4852) through the route of Abu Salih, from Ibn Umar, as a raised report with the same wording, adding, Abu Salih said, "I said to Ibn Umar, 'What about four?' He said, 'It will not harm you.'"This hadith applies when they are three. But when they are more than three, there is no harm in whispering and private conversation, as Ibn said.