مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3932
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdur-Rahman bin 'Abdullah bin Mas'ood, from his father who said: The Messenger of Allah (ﷺ) said: “For a Muslim to fight his brother is kufr and reviling him is an evil action."
Musnad Ahmad 3957A saheeh hadeeth]