Hadith 3776
Chapter 50: Two should not converse to the exclusion of a third - كتاب الأدب
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 .
It was narrated that Ibn 'Umar said:
"The Messenger of Allah(ﷺ) forbade two to converse (privately) to the exclusion of a third."
Reference : Sunan Ibn Majah 3776
In-book reference : Book 33, Hadith 120