Habib bin Abi Thabit حبيب بن أبي ثابت قيس · The Book on Fasting

the Messenger of Allah said: "Whoever breaks the fast during Ramadan without an allowance or illness, then if he fasted for all time, his fasting would not make up for it."

The Messenger of Allah said: "The most virtuous fast is the fast of my brother Dawud. He would fast a day, and not fast (the next) day. He would not flee at the time of engagement (with the enemy)."