Muwatta Malik, Book 56, Hadith 14 (USC-MSA) ⓘ
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."
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 " .
also referenced under: Book 56, Hadith 1827 (In-Book)