An Nasai: Hadith 2404

"I entered upon 'Abdullah bin 'Amr and said: O uncle, tell me what the Messenger of Allah said to you. He said: O son of my brother, I had resolved to strive very hard until I said: I will fast for the rest of my life and I will read the whole Quran every day and night. The Messenger of Allah heard about that, and came in to me in my house, and said: I have heard that you said, I will fast for a lifetime and will read the Quran. I said: I did say that, O Messenger of Allah. He said; Do not do that. Fast three days of each month. I said: I am able to do more than that. He said: fast two days of each week, Monday and Thursday. I said: I am able to do more than that. He said; Observe the fast of Dawud, peace be upon him, for it is the best kind of fasting before Allah; one day fasting, and one day not fasting. And when he made a promise he did not break it, and when he met (the enemy in battle) he did not flee." 'Ata said: "someone who heard him told me that Ibn 'Umar (said) that the Prophet said: 'Whoever fasts every day of his life, then he has not fasted."