Musnad Ahmad 25:422

Mu'ammar ibn Sulayman—meaning al-Raqqi—narrated to us, from al-Hajjaj, from 'Ikrimah, from Ibn 'Abbas, from the Prophet, peace and blessings of Allah be upon him, who said: "There is no marriage except with a guardian (wali), and the ruler is the guardian of the one who has no guardian."