مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4099
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdullah bin Mas'ood that The Prophet (ﷺ) said: "Trading insults with a Muslim is an evil action and fighting him is kufr.”
Musnad Ahmad 4126Its isnad is saheeh, Muslim (64)]