Narrated Qatadah Ibn Malhan al-Qaysi:
“The Messenger of Allah (may Allah bless him and grant him peace) 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.””
[Abu Dawud]
Was Salam
Abu Zaynah
Islamopedia 