Musnad Ahmad 25:1616

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, he said: 'Ubayd Allah ibn Abi Yazid informed me that he heard Ibn 'Abbas saying: "I did not know the Messenger of Allah, may Allah bless him and grant him peace, to be intent on fasting any day seeking its excellence over others, except for this day, the day of 'Ashura', or Ramadan." Rawh said: "Or the month of Ramadan."