Sayyiduna Abdullah bin Amr (may Allah be pleased with him) narrates that I presented myself before the Messenger of Allah (peace and blessings be upon him) and asked about fasting. The Prophet (peace and blessings be upon him) said: "Fast one day in every month and you will receive the reward for the remaining days as well." I said, "I have the strength to do more than that." The Prophet (peace and blessings be upon him) said: "Fast two days in every month and you will receive the reward and recompense for the remaining days as well." I said, "Indeed, I have the strength to do more than that." The Prophet (peace and blessings be upon him) said: "Fast three days in every month and you will also receive the reward for the remaining days." I again said, "I find myself capable and strong enough to do more than that." The Prophet (peace and blessings be upon him) said: "Fast four days and you will also receive the reward for the remaining days." He said, "Indeed, I find myself capable and strong enough to do more than that." So the Messenger of Allah (peace and blessings be upon him) said: "The most beloved fasts are the fasts of Dawud (peace be upon him); he would fast one day and leave (not fast) the next day."
Hadith Referenceصحيح ابن خزيمه / 2106
Hadith Takhrijصحيح مسلم
Hadith 2107
In the narration of Sayyiduna Abdullah bin Amr رضي الله عنه, it is stated: Observe fasting like the fasts of Dawud عليه السلام, for they are the best and most moderate fasts in the sight of Allah Almighty.
Hadith Referenceصحيح ابن خزيمه / 2107
Hadith 2108
The best of fasts are the fasts of Prophet Dawud (peace be upon him). I have mentioned the chains of these narrations in Kitab al-Kabir.