Sayyiduna Usamah bin Zayd (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "Do not turn back into disbelievers after me, striking the necks of one another."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a narrator whom I do not know.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الفتن - أعاذنا الله منها / 12297