Muwatta Malik, Book 56, Hadith 1826 (In-Book) ⓘ

Malik related to me from Abdullah ibn Dinar saying, "Abdullah ibn Umar and I were at the house of Khalid ibn Uqba who was away at the market. A man came who wanted to speak to Abdullah ibn Umar and I was the only other person present Abdullah ibn Umar called another man so that we were four and said to me and the man whom he had called, 'Go a little way off together, because I heard the Messenger of Allah, may Allah bless him and grant him peace, say, "Two do not converse secretly to the exclusion of another.' "'

وَحَدَّثَنِي مَالِكٌ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، قَالَ كُنْتُ أَنَا وَعَبْدُ اللَّهِ بْنُ عُمَرَ، عِنْدَ دَارِ خَالِدِ بْنِ عُقْبَةَ الَّتِي بِالسُّوقِ فَجَاءَ رَجُلٌ يُرِيدُ أَنْ يُنَاجِيَهُ وَلَيْسَ مَعَ عَبْدِ اللَّهِ بْنِ عُمَرَ أَحَدٌ غَيْرِي وَغَيْرُ الرَّجُلِ الَّذِي يُرِيدُ أَنْ يُنَاجِيَهُ فَدَعَا عَبْدُ اللَّهِ بْنُ عُمَرَ رَجُلاً آخَرَ حَتَّى كُنَّا أَرْبَعَةً فَقَالَ لِي وَلِلرَّجُلِ الَّذِي دَعَاهُ اسْتَأْخِرَا شَيْئًا فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏ "‏ لاَ يَتَنَاجَى اثْنَانِ دُونَ وَاحِدٍ ‏" ‏ ‏.‏