مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3623
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdullah (رضي الله عنه) that The Prophet (ﷺ) said: "Trading insults with a Muslim is an evil action and fighting him is kufr.” I said to Abu Wa'il: Did you hear that from Abdullah? He said: Yes.
Musnad Ahmad 3647Its isnad is saheeh, al-Bukhari (48) and Muslim (64)]