Musnad Ahmad 27:700

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah, Nafi' informed me, from 'Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: And 'Abd al-Rahman, from Malik, from Nafi', from Ibn 'Umar, from the Prophet (peace and blessings of Allah be upon him) who said: "Whoever bears arms against us is not one of us."