مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4066
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdullah that The Prophet (ﷺ) said: “If you are three, two should not converse (privately) to the exclusion of their companion, because that makes him sad."
Musnad Ahmad 4093Its isnad is saheeh, Muslim (2184)]