Musnad Ahmad 90:2

Ya'qub narrated to us, 'Abd al-'Aziz ibn al-Muttalib al-Makhzumi narrated to us, from his brother al-Hakam ibn al-Muttalib, from his father, from Quhayd al-Ghifari, who said: A questioner asked the Messenger of Allah, may Allah bless him and grant him peace, and said: "If an aggressor attacks me?" The Messenger of Allah, may Allah bless him and grant him peace, said: "Remind him and command him to be reminded three times. If he refuses, then fight him. If he kills you, then you are in Paradise, and if you kill him, then he is in the Fire."