Musnad Ahmad 333:7

Narrated to us Abd al-Rahman ibn Mahdi, from Ikrimah ibn Ammar, from Iyas ibn Salamah ibn al-Akwa, from his father, who said: "Our watchword on the night we were attacked by Hawazin, while Abu Bakr al-Siddiq was in command over us by the order of the Messenger of Allah, may Allah bless him and grant him peace, was: 'Amit, Amit' (Kill, Kill)." "And I killed with my own hand that night seven men from the households."