Musnad Ahmad 27:971

'Affan narrated to us, Shu'bah narrated to us, Ibn Abi Najih said: He informed me, he said: I heard my father narrating from a man, from Ibn 'Umar, that he asked him about fasting the day of 'Arafah. He said: "We went out with the Messenger of Allah, peace and blessings of Allah be upon him, and he did not fast it. And with Abu Bakr, and he did not fast it. And with 'Umar, and he did not fast it. And with 'Uthman, and he did not fast it. And I do not fast it, and I do not command you [to do so], nor do I forbid you. If you wish, fast it, and if you wish, do not fast it."