Riyad as-Salihin 1598
The Book of the Prohibited actions
Prohibition of two Holding Secret Counsel to the Exclusion of Conversing together a Third
Ibn 'Umar (May Allah be pleased with them) said: the Messenger of Allah (ﷺ) said, "In the presence of three people, two should not hold secret counsel, to the exclusion of the third." [Al- Bukhari and Muslim]. In Abu Dawud, Abu Salih related: I asked Ibn 'Umar: "What if there are four people." He said, "There is no harm in that." Malik reported in Al-Muwatta that 'Abdullah bin Dinar related: Ibn 'Umar and I were together in Khalid bin 'Uqbah's house which was situated in the market place. A man came to consult Ibn 'Umar. None besides me was present. Ibn 'Umar called another man in and we became four and said to me and the man he had called: Move away a bit because I have heard the Messenger of Allah (ﷺ) saying, "The two people should not hold secret counsel together excluding the third."