Ibn Abbas was asked about fasting on the Day of Ashura. He replied, "I do not know of any day that the Messenger of Allah ﷺ fasted seeking its merit over other days except this day, nor any month except this month," meaning Ramadan.
عن ابن عباس، وسئل عن صيام يوم عاشوراء؟ فقال: ما علمت ان رسول الله ﷺ صام يوما يطلب فضله على الأيام إلا هذا اليوم، ولا شهرا إلا هذا الشهر- يعني رمضان-
Bukhari narrated it in the Book of Fasting (2006), and Muslim in the Book of Fasting (1132), both through the route of Sufyan bin Uyayna, from Ubaydullah bin Abu Yazid, from Ibn Abbas, with the same chain, mentioning it. The wording given is Muslim's.