He was asked about fasting the day of Ashura, and 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.
وسئل عن صيام يوم عاشوراء؟ فقال: ما علمت أن 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 Sufyan bin Uyayna, from Ubaydullah bin Abu Yazid, from Ibn Abbas, who mentioned it. The wording given here is Muslim's.