Musnad Ahmad 27:631

Isma'il ibn Ibrahim and Sufyan ibn 'Uyaynah narrated to us, they said: Ibn Abi Najih narrated to us, from his father, who said: Ibn 'Umar was asked about fasting the day of 'Arafah, and he said: "I performed Hajj with the Prophet, may Allah bless him and grant him peace, and he did not fast it." "And I performed Hajj with Abu Bakr and he did not fast it, and I performed Hajj with 'Umar and he did not fast it, and I performed Hajj with 'Uthman and he did not fast it." "And I do not fast it, nor do I command it, nor do I forbid it." And Sufyan said once: "From the one who asked Ibn 'Umar."