Musnad Abu Ya'la al-Mawsili 50:2

Yahya ibn Ayyub narrated to us, Abbad ibn Abbad narrated to us, from Mujalid ibn Sa'id, from Qays ibn Abi Hazim, from al-Sunabihi, who said: The Messenger of Allah (peace be upon him) said: "I will boast of your great numbers before the nations, so do not return after me as disbelievers, striking the necks of one another."