«كان يصوم من غرة كل شهر ثلاثة أيام وقلما كان يفطر يوم الجمعة»
It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would fast three days from the beginning of every month, and it was rare that he would not be fasting on Friday."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4972