Musnad Ahmad 25:1354

Waki’ narrated to us, Ibn Abi Dhi’b narrated to us, from al-Qasim ibn ‘Abbas, from ‘Abdullah ibn ‘Umayr, a freed slave of Ibn ‘Abbas, from Ibn ‘Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If I remain until next year, I will surely fast the ninth day."