It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said during the Farewell Hajj: "Do not turn back after me into disbelievers, striking one another's necks."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13477