Jami` at-Tirmidhi 753
جامع الترمذي#753SahihAishah narrated:"Ashura was a day that the Quraish used to fast during Jahiliyyah, and the Messenger of Allah used to fast it. But when (the fast of) Ramadan became obligatory, the Ramadan was the required and Ashura was left. So whoever wanted to, he fasted it, and whoever wanted to, he left it
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Bashar Awad MaaroufSahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 8:72