«كان يصوم من الشهر السبت والأحد والاثنين ومن الشهر الآخر الثلاثاء والأربعاء والخميس»
It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would fast on Saturday, Sunday and Monday in one month, and on Tuesday, Wednesday and Thursday in the next month."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4971