Musnad Ahmad 5:179

Waki’ narrated to us, Sufyan narrated to us, from Habib, from Abu Wa’il, from Abu al-Hayyaj al-Asadi, who said: Ali said to me: "Shall I send you on that which the Messenger of Allah (peace and blessings of Allah be upon him) sent me on? 'Do not leave any image without effacing it, and do not leave any elevated grave without leveling it.'"