مُسْنَدُ أَحْمَدَ
Musnad Ahmad 1519
Musnad Abu Ishaq Sa'd bin Abi Waqqas
Sa"d bin Abi Waqqas narrated: The Messenger of Allah (ﷺ) said: "Killing a believer is kufr (disbelief) and reviling him is evildoing. It is not permissible for a Muslim to shun his brother for more than three days."
Musnad Ahmad 1519Hasan (Darussalam), and the hadeeth is Sahih]