مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2104
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Ibn 'Abbas (رضي الله عنه) that the Messenger of Allah (ﷺ) said: "If I live - Rawh said: If I am still sound - until next year, I will definitely fast the ninth," meaning the day before ‘Ashooraʻ.
Musnad Ahmad 2106lts isnad is qawi]