The Messenger of Allah (ﷺ) used to command us to fast the days of the white (nights): thirteenth, fourteenth and fifteenth of the month. He said: This is like keeping perpetual fast.
Sunan Abi Dawud Book 14, Hadith 137
← Previous Hadith The Messenger of Allah (ﷺ) said to me: The fast most liked by Allah is the one observed by Dawud (David), and the prayer dearer to Allah is the one offered by Dawud (David): he would sleep half the night, and stand (in prayer) one-third of it, and sleep one-sixth of it. He would go without fasting one day, and fast the other day.

Sunan Abi Dawud Book 14, Hadith 136 Next Hadith → The Messenger of Allah (ﷺ) used to fast three days every month.

Sunan Abi Dawud Book 14, Hadith 138