مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3537
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah bin Mas'ood (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: "If you are three, two should not converse (privately) to the exclusion of their companion, because that makes him sad."
Musnad Ahmad 3560Its isnad is saheeh, Muslim (2184)]