Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 126
It is narrated on the authority of Abdullah b. Umar that the Apostle (may peace and blessings be upon him) observed on the occasion of the Farewell Pilgrimage Woe unto you distress unto you! Don't turn back as unbelievers after me by striking the necks of one another.