Sahih Bukhari
Fasting
Volume: 003 / Book Number: 031 / Hadith Number: 208
Narrated 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?"