مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2856
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Sadaqah ad-Dimashqi said: A man came to Ibn ‘Abbas (رضي الله عنه) and asked him about fasting. He said: The Messenger of Allah (ﷺ) used to say: “One of the best kinds of fasting is the fast of my brother Dawood. He used to fast one day and not fast the next day.”
Musnad Ahmad 2876Its isnad is da’eef jiddan]