Sunan Ibn Majah
3942
Sahih+3

It was narrated from Jarir bin ‘Abdullah that the Messenger of Allah (ﷺ) said, during the Farewell Pilgrimage:“Make the people pay attention.” Then he said: “Do not turn back into disbelievers after I am gone, striking one another’s necks.”

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi