Sunan at-Tirmithiy كتاب الصوم باب ما جاء فى صوم ثلاثة أيام من كل شهر
The Messenger of God, peace and blessings of God be upon him, said to Abū-Tharr, "Abū-Tharr, if you are to fast three days of a month, then fast the thirteenth, the fourteenth, and the fifteenth (1)." Footnotes: (1) These are the days of the nights known collectively as the "White Nights". Click on ḥadīth to read more