مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2834
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
Ibn ‘Abbas (رضي الله عنه) said: I never knew the Messenger of Allah (ﷺ) to seek out any day to fast, seeking its virtue over other days, except this day, the day of ‘Ashoora’, or the month of Ramadan.
Musnad Ahmad 2854Its isnad is saheeh, al-Bukhari (2006) and Muslim (1132)]