مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3452
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
'Ubaidullah bin Abi Yazeed narrated that he heard Ibn 'Abbas (رضي الله عنه) say: I did not see the Messenger of Allah (ﷺ) making sure to fast any day, seeking its virtue and superiority over other days, apart from this day - meaning ‘Ashoora' - or Ramadan.
Musnad Ahmad 3475Its isnad is saheeh, al-Bukhari (2006) and Muslim (1132)]