مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4151
Musnad of Abdullah bin Mas'ood
It was narrated from ‘Abdullah (رضي الله عنه) from the Prophet (ﷺ) that He said: “Trading insults with a Muslim is an evil action and fighting him is kufr.”
Musnad Ahmad 4178lts isnad is saheeh, Muslim (64)]