ENOfficial
'Abdullah reported that the Prophet, 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 for that would grieve him."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عُمَرُ بْنُ حَفْصٍ قَالَ: حَدَّثَنِي أَبِي، قَالَ: حَدَّثَنَا الأَعْمَشُ قَالَ: حَدَّثَنِي شَقِيقٌ، عَنْ عَبْدِ اللهِ قَالَ: قَالَ النَّبِيُّ صلى الله عليه وسلم: إِذَا كُنْتُمْ ثَلاَثَةً فَلاَ يَتَنَاجَى اثْنَانِ دُونَ الثَّالِثِ، فَإِنَّهُ يُحْزِنُهُ ذَلِكَ.
sunnah.com