Sahih Bukhari Volume 003, Book 031, Hadith Number 208.

Narated By Alqama : I asked 'Aisha "Did Allah s Apostle, use to choose some special days (for fasting)?" She replied, "No, but he used to be regular (constant) (in his service of worshipping). Who amongst you can endure what Allah's Apostle used to endure?"