Musnad Abu Ya'la al-Mawsili 153:181

Harun ibn Ma'ruf narrated to us, Sufyan narrated to us, from Ibn Abi Najih, from his father, who said: Ibn Umar was asked about fasting on the day of Arafah. He said: "I performed Hajj with the Prophet, may Allah bless him and grant him peace, and he did not fast it. I performed Hajj with Abu Bakr and Umar, and they did not fast it. I performed Hajj with Uthman, and he did not fast it. As for me, I do not fast it, nor do I command it, nor do I forbid it."