Sahih Bukhari Volume 003, Book 031, Hadith Number 197.

Narated By 'Abdullah bin 'Amr : Allah's Apostle was informed that I had taken an oath to fast daily and to pray (every night) all the night throughout my life (so Allah's Apostle came to me and asked whether it was correct): I replied, "Let my parents be sacrificed for you! I said so." The Prophet said, "You can not do that. So, fast for few days and give it up for few days, r ray and sleep. Fast three days a month as the reward of good deeds is multiplied ten times and that will be equal to one year of fasting." I replied, "I can do better than that." The Prophet said to me, "Fast one day and give up fasting for a day and that is the fasting of Prophet David and that is the best fasting." I said, "I have the power to fast better (more) than that." The Prophet said, "There is no better fasting than that."