Sunan Ibn Majah, Volume 5, Book 33, Hadith 3776 (DarusSalam) ⓘ
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: Hadith 3776 (Reference) & Book 33, Hadith 121 (In-Book)