The Messenger of Allah ﷺ said, "When you are three, two should not converse privately, leaving out their companion."
حدثنا إسحاق بن يوسف حدثنا الأعمش عن أبي صالح
قال رسول الله ﷺ إذا كنتم ثلاثة فلا يتناجين اثنان دون صاحبهما
Its isnad is sahih. Ishaq bin Yusuf is Azraq, whose reliability was already established at 943, and we add here that Ibn Main and Ajli, among others, confirmed his reliability. Ahmad was asked, "Is Ishaq Azraq trustworthy?" and he said, "Yes, by Allah, he is trustworthy." Khatib said in Tarikh Baghdad 6:319, "He was among the trustworthy and reliable, and one of Allah's righteous servants," and mentioned that he heard from Amash. Bukhari gave his biography in al-Kabir 1/1/406 and stated explicitly that he heard from Amash, and mentioned that he died in the year 194.Abu Salih is Zakwan Samman. The hadith is an abridged version of 6085, and see also 6225.