Hadith 518070
Chapter 56: Speech - كتاب الكلام
وَحَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا كَانَ ثَلاَثَةٌ فَلاَ يَتَنَاجَى اثْنَانِ دُونَ وَاحِدٍ " .
Malik related to me from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, "Two must not converse secretly to the exclusion of another person."
Reference Muwatta Malik 518070
USC-MSA web (English) reference : Book 56, Hadith 14