Voluntary Fasts
Hazrat Anas (ra) relates that during the lifetime of the Holy Prophet (sa), Hazrat Abu Talha (ra) would refrain from voluntary fasting as he wanted to preserve his strength when engaging in Jihad. Hazrat Anas (ra) further relates that after the Holy Prophet (sa) passed away, he observed that Hazrat Abu Talha (ra) kept fasts every single day except on Eid-ul-Fitr and Eid-ul-Adha. In other words, he started regularly observing fast after this.
(Sahih al-Bukhari, Kitabul Jihad wa as-Siyar, Bab Man Ihktara al-Ghazwa alaa as-Saum, Hadith 2828)