Musnad Ahmad 27:668

Isma'il narrated to us, Ibn Abi Najih informed 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."