Sunan Ibn Majah, Hadith 3776 ⓘ
It was narrated that Ibn 'Umar said: "The Messenger of Allah forbade two to converse (privately) to the exclusion of a third."
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنِ ابْنِ عُمَرَ، قَالَ نَهَى رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ أَنْ يَتَنَاجَى اثْنَانِ دُونَ الثَّالِثِ .
also referenced under: Book 33, Hadith 121 (In-Book) & Volume 5, Book 33, Hadith 3776 (DarusSalam)