Kitaab-us-Siyaam [Book of Fasting] starts today.
Alhamdulillah, perfect timing with Ramadan right around the corner. :)
Volume 3, Book 31, Number 115:
Narrated Talha bin ‘Ubaid-Ullah (radiallaahu `anhu):
A bedouin with unkempt hair came to Allah’s Apostle (sallallaahu `alayhi wasallam) and said, “O Allah’s Apostle (sallallaahu `alayhi wasallam)! Inform me what Allah has made compulsory for me as regards the prayers.” He replied: “You have to offer perfectly the five compulsory prayers in a day and night (24 hours), unless you want to pray Nawafil.” The bedouin further asked, “Inform me what Allah has made compulsory for me as regards fasting.” He replied, “You have to fast during the whole month of Ramadan, unless you want to fast more as Nawafil.” The bedouin further asked, “Tell me how much Zakat Allah has enjoined on me.” Thus, Allah’s Apostle (sallallaahu `alayhi wasallam) informed him about all the rules (i.e. fundamentals) of Islam. The bedouin then said, “By Him Who has honored you, I will neither perform any Nawafil nor will I decrease what Allah has enjoined on me. Allah’s Apostle (sallallaahu `alayhi wasallam) said, “If he is saying the truth, he will succeed (or he will be granted Paradise).”
This indicates that the fasting that is obligatory is fasting in the month if Ramadaan. With regard to fasting on other occasions, it is not a sin if one does not do it.
But we should not ignore fasting on other occasions such as ‘Ashoora’, ‘Arafah, six days of Shawwaal and so on, because of the great reward that this brings and the great status it has. Moreover observing naafil fasts makes up for any shortfall that there may be in the obligatory fasts.
It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say:
The first of his deeds for which a person will be brought to account on the Day of Resurrection will be his prayer. If it is sound he will have succeeded, and if it is lacking he will be a loser. If anything is lacking from his obligatory prayers, the Lord will say, ‘Look and see if My slave has any voluntary prayers, and use them to make up what is lacking from his obligatory prayers.’ Then all his deeds will be dealt with in like manner.
Narrated by al-Tirmidhi, no. 413; al-Nasaa’i no. 465. Classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
This indicates that observing naafil fasts will make up for any lack in the obligatory fasts.
P.S. you’l note that the Prophet (sallallaahu `alayhi wasallam) gave the Bedouin (questioner in the Hadith) option to pray and fast voluntarily as well, instead of only telling him about the obligatory stuff , although the answer would have sufficed. This shows that he was trying to motivate him to do more than just the compulsory acts – the bare minimum.