Musnad Ahmad 27:962

Mu'ammal narrated to us, Sufyan narrated to us, from Isma'il ibn Umayyah, from Nafi', who said: Ibn 'Umar was asked about fasting the day of 'Arafah? He said: "The Prophet, peace and blessings of Allah be upon him, did not fast it, nor did Abu Bakr, nor 'Umar, nor 'Uthman." Waki' narrated to us, from Sufyan, from Isma'il ibn Umayyah, from a man, from Ibn 'Umar, who said: "The Prophet, peace and blessings of Allah be upon him, did not fast it, nor did Abu Bakr, nor 'Umar, nor 'Uthman," meaning the day of 'Arafah.