The Prophet ﷺ said, "When you are three, two should not converse privately to the exclusion of their companion."We asked, "What if they are four?"He replied, "Then it does no harm."
حدثنا يحيى عن الأعمش عن أبي صالح
عن النبي ﷺ قال إذا كنتم ثلاثة فلا ينتجي اثنان دون صاحبهما قال قلنا فإن كانوا أربعا قال فلا يضر
Its isnad is sahih. Abu Salih is al-Samman, whose name was Zakwan. This hadith is the one we referred to under 4504 as being narrated by Abu Dawud, who related it at 1:414 through Isa bin Yunus, from Amash, from Abu Salih. Abu Dawud's version makes clear that the one who asked, "What if there were four?" was Abu Salih himself, for it states, Abu Salih said, "So I said to Ibn Umar, 'What about four?'" He said, "It will not harm you." See also 4564 and 4664.