He was asked about fasting on Ashura. He said, "I do not know of the Prophet ﷺ fasting any day, seeking its merit over other days, except this day," meaning the month of Ramadan and the day of Ashura.
أخبرنا قتيبة قال حدثنا سفيان عن عبيد الله أنه
وسئل عن صيام عاشوراء قال ما علمت النبي ﷺ صام يوما يتحرى فضله على الأيام إلا هذا اليوم يعني شهر رمضان ويوم عاشوراء
Bukhari narrated it in the Book of Fasting, Chapter on Fasting the Day of Ashura, no. 2006, and Muslim in the Book of Fasting, Chapter on Fasting the Day of Ashura, no. 1132, from the hadith of Sufyan bin Uyayna through this chain, and it is in Al-Kubra, no. 2679.Ubaydullah is the son of Abu Yazid.