مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2057
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas said: The Messenger of Allah (ﷺ) sent word to the people of a village four parasangs away or he said: two parasangs - on the day of ‘Ashoora', telling everyone who had eaten not to eat for the rest of the day, and telling those who had not eaten to complete the day tasting.
Musnad Ahmad 2058Saheeh, because of corroborating evidence; this is a da'eef