مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4315
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.” Zubaid said: I said to Abu Wa'il twice: Did you hear it from ‘Abdullah from the Prophet (ﷺ)? He said Yes.
Musnad Ahmad 4345Its isnad is saheeh]