Musnad Ahmad 28:313

Waki’ narrated to us, Sufyan narrated to us, from Habib ibn Abi Thabit, from Abu al-Abbas al-Makki, 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 fasting of my brother David; he used to fast one day and break his fast one day, and he would not flee when he encountered [the enemy]." And the Messenger of Allah (peace and blessings of Allah be upon him) said: "He has not fasted who fasts forever."