From Sahih Bukhari (Hadith 1987):
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 Messenger (ﷺ) used to endure?
Reference: Bukhari 1987