Ibn Umar reported Messenger of Allah ﷺ as saying: When there are three (persons), two should not converse secretly between themselves to the exclusion of the (third) one.

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ
اللَّهِ ﷺ قَالَ
" إِذَا كَانَ ثَلاَثَةٌ فَلاَ يَتَنَاجَى اثْنَانِ دُونَ وَاحِدٍ " .