مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3816
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah bin Mas'ood (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: "He is not a believer who maligns People or curses people a great deal or is foulmouthed and rude.”
Musnad Ahmad 3839A saheeh hadeeth, but this is a munkar isnad]