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 theMessenger of Allah, may Allah bless him and grant him peace, said,"Two must not converse secretly to the exclusion of another person." Translation Not Available

Book reference: Muwatta Imam Malik Book 56 Hadith 1827
Web reference: Muwatta Imam Malik Book 56 Hadith 14