Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
ahmad:4685 – Yaḥyá > al-Aʿmash > Abū Ṣāliḥ > Ibn ʿUmar

[Machine] On the authority of the Prophet ﷺ, he said, "When you are three, then two should not converse privately, excluding their companion." They said, "What if they are four?" He said, "It will not harm."  

أحمد:٤٦٨٥ – حَدَّثَنَا يَحْيَى عَنِ الْأَعْمَشِ عَنْ أَبِي صَالِحٍ عَنِ ابْنِ عُمَرَ

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا كُنْتُمْ ثَلَاثَةً فَلَايَنْتَجِي اثْنَانِ دُونَ صَاحِبِهِمَا قَالَ قُلْنَا فَإِنْ كَانُوا أَرْبَعًا؟ قَالَ فَلَا يَضُرُّ  

See similar narrations below:

Collected by Aḥmad, Suyūṭī
ahmad:6264 – Isḥāq b. Yūsuf > al-Aʿmash > Abū Ṣāliḥ > Ibn ʿUmar

"When you are three, two should not converse (privately) to the exclusion of their companion, because that makes him sad." (Using translation from Ibn Mājah 3775)   

أحمد:٦٢٦٤ – حَدَّثَنَا إِسْحَاقُ بْنُ يُوسُفَ حَدَّثَنَا الْأَعْمَشُ عَنْ أَبِي صَالِحٍ عَنِ ابْنِ عُمَرَ قَالَ

قَالَ رَسُولُ اللهِ ﷺ إِذَا كُنْتُمْ ثَلَاثَةً فَلَا يَتَنَاجَيَنَّ اثْنَانِ دُونَ صَاحِبِهِمَا