مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4319
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdur-Rahman bin Yazeed said: al-Ash'ath bin Qais entered upon ‘Abdullah and he was eating lunch. He said: O Abu Muhammad, come and eat lunch. He said: Is it not the day of ‘Ashoora’? He said: What about it? Rather it is a day that the Messenger of Allah (ﷺ) used to fast before (the command to fast) Ramadan was revealed; when (the command to fast) Ramadan was revealed, it was given up.
Musnad Ahmad 4349Its isnad is saheeh, al-Bukhari (4503) and Muslim (1127)]