مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3792
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdur-Rahman bin ‘Abdullah. from his father, that The Prophet (ﷺ) said: Do not go back to kufr after I am gone, striking one another's necks."
Musnad Ahmad 3815Saheeh. This is a saheeh isnad according to those who think