Is there any special reward for observing nafl fasts?
Please, add reference from the Quran and Sunnah.
As far as I know nafl fasts is not mentioned in Qur'an directly but this ayah is general on fast:
"O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous -"
You can look hadith references in this article: VOLUNTARY FASTS