Musnad Ahmad 25:1017

Abu al-Nadr narrated to us, he said: al-Faraj ibn Fudalah narrated to us, from Abu Haram, from Sadaqah al-Dimashqi, he said: A man came to Ibn Abbas, asking him about fasting. He said: The Messenger of Allah, may Allah bless him and grant him peace, used to say: "Indeed, among the best of fasting is the fasting of my brother David; he used to fast one day and break his fast one day."