Hadith 2449
Narrated Qatadah Ibn Malhan al-Qaysi: 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.
Hadith 2450
Narrated Abdullah ibn Masud: The Messenger of Allah ﷺ used to fast three days every month.