Musnad Ahmad 28:58

Waki’ narrated to us, Sufyan and Mis’ar narrated to us, from Habib ibn Abi Thabit, from Abu al-Abbas, from Abdullah ibn Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best of fasting is the fast of my brother Dawud (peace be upon him); he used to fast one day and break his fast one day, and he would not flee when he met [the enemy]."