Malik related to me from Abdullah ibn Dinar saying, "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 do not converse
secretly to the exclusion of another.' "'
Muwatta Malik Book 56, Hadith 13