Then observe the fast of David (peace be upon him). Observe fast one day and break it (on the other) day. And he ('Abdullah b. 'Amr) used to say: Would that I had availed myself of this concession.
Sahih Muslim Book 13, Hadith 249
← Previous Hadith Observe fast for a day and there would be reward for you for the rest (of the days). He said: I am capable of doing more than this. He then said: Observe fast for two days, and there would be reward for you for the rest (of the days). He said: I am capable of doing more than this. He (the Holy Prophet) said: Observe fast for three days and there would be reward for you for the rest of the days. He said: I am capable of doing more than this, whereupon he said: Observe fast for four days and there would be reward for you for the rest of the days. He said: I am capable of doing more than this. Thereupon he said: Then observe fast (which is the) best in the eye of Allah, the fast of David (peace be upon him) ; he used to observe fast one day and break on the other day.

Sahih Muslim Book 13, Hadith 248 Next Hadith → Yes I said to her: Which were (the particular) days of the month on which he observed fast? She said: He was not particular about the days of the month on which to observe fast.

Sahih Muslim Book 13, Hadith 250