It is permissible to break a voluntary fast
15-6-2014 | IslamWeb
Question:
What is the textual evidence of the permissibility of breaking a voluntary fast?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Islam encourages Muslims to perform voluntary righteous deeds so as to obtain much rewards and draw close to Allah. If a person starts a voluntary fast, then he is not obligated to complete it, or make up for it should he break it. ‘Aa’ishah 
said to the Prophet, sallallaahu ‘alayhi wa sallam, “O Messenger of Allah! We have been offered a Hays (which is a type of food) as a gift.” The Prophet, sallallaahu ‘alayhi wa sallam, asked ‘Aa’ishah to bring the food near to him and ate, even though he was (voluntarily) fasting that day. [Muslim]
According to An-Nasaa’i's narration of the same Hadeeth, the Prophet, sallallaahu ‘alayhi wa sallam, added: “The one who has decided to start a voluntary fast is like the one who has decided to give a voluntary charity. if he wishes he may give it, and if he wishes he may withhold it." [An-Nasaa’i]
The Prophet, sallallaahu ‘alayhi wa sallam, also says: “The one observing a voluntary fast has full authority whether to complete the fast or break it.” [Ahmad and At-Tirmithi] In his book Kashf Al-Khafaa’, Al-‘Ajlooni 
judged this Hadeeth as having an authentic chain of transmitters. Al-Albaani also considered this Hadeeth authentic.
However, if a person starts a voluntary fast, then it is highly recommended to fulfill it, compliant with the statement of Allah (which means): {…and do not invalidate your deeds.}[Quran 47: 33]