مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4079
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah said: The Messenger of Allah (ﷺ) said: "If you are three, two should not converse (privately) to the exclusion of one, because that makes him sad.”
Musnad Ahmad 4106Its isnad is saheeh, al-Bukhari (6290) and Muslim (2184)]