Sunan an Nasai
2370
Sahih+2

It was narrated from 'Ubaidullah that:he heard Ibn 'Abbas, when he was asked about the fast of 'Ashura' say: "I do not know that the Prophet fasted any day because of its virtue, except this day" meaning the month of Ramadan and the day of Ashura

SahihAbu Ghuddah
DaifAl-Albani