Musnad Ahmad 5:672

Yazid narrated to us, Ash'ath ibn Siwar informed us, from Ibn Ashwa', from Hanash ibn Abi al-Mu'tamir, that Ali sent his chief of police and said: "I am sending you for that which the Messenger of Allah, peace and blessings of Allah be upon him, sent me for: 'Do not leave any grave without leveling it, and do not leave any statue without destroying it.'"