Riyad as-Salihin (The Meadows of the Righteous)
by Imam Nawawi
281. Chapter: On the prohibition against two people conversing to the exclusion of a third without his permission except in case of need. It is the same if two people speak in a language which he does not understand.
Allah Almighty says, "Conferring in secret is from Shaytan." (58:10)
1598. Ibn 'Umar reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "When there are three together, two should not speak privately to the exclusion of the third." [Agreed upon]
Abu Dawud related it and added that Abu Salih said, "I asked Ibn 'Umar, 'What about four?' He replied. 'It will not harm you.'"
Malik reported in al-Muwatta' that 'Abdullah ibn Dinar said, "'Abdullah ibn 'Umar and I were at the house of Khalid ibn 'Uqba who was away at the market. A man came who wanted to speak to 'Abdullah ibn Umar and I was the only other person present. 'Abdullah ibn Umar called another man so that we were four and said to me and the man whom he had called, 'Go a little way off together because I heard the Messenger of Allah, may Allah bless him and grant him peace, say, "Two should not converse to the exclusion of a third." ' "
1599. Ibn Mas'ud reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "When you are three together, two should not speak together privately apart from the other one until you come together with other people because that might upset him." [Agreed upon]