Musnad Ahmad 1007:100

'Affan narrated to us, Hammam narrated to us, he said: 'Ata' ibn Abi Rabah was asked, and I was a witness, about the virtue of fasting the day of 'Arafah. He said: "This has come from your side, O people of Iraq. Abu al-Khalil narrated it to me, from Harmalah ibn Iyas, from Abu Qatadah, that the Prophet, peace and blessings of Allah be upon him, said"—a word resembling the equivalent of that—"Fasting 'Arafah is [equivalent to] fasting two years, and fasting 'Ashura' is [equivalent to] fasting one year."