Two should not converse to the exclusion of a third Al-Adab Al-Mufrad 1168
Arabic hadith text
حَدَّثَنَا إِسْمَاعِيلُ قَالَ: حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللهِ، أَنَّ رَسُولَ اللهِ صلى الله عليه وسلم قَالَ: إِذَا كَانُوا ثَلاَثَةً، فَلاَ يَتَنَاجَى اثْنَانِ دُونَ الثَّالِثِ.
Translation
'Abdullah reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "When there are three people, two should not converse together to the exclusion of the third."