Malik related to me from Nafi from Abdullah ibn Umar that the Messenger of Allah ﷺ said, "Two must not converse secretly to the exclusion of another person."
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا كَانَ ثَلاَثَةٌ فَلاَ يَتَنَاجَى اثْنَانِ دُونَ وَاحِدٍ