Maliks Muwatta Book 56, Hadith Number 13.

Section : Two People Conversing to the Exclusion of Another.

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.'”