Fasting: Intentionally Invalidating a Vowed Fast on an Assigned Day
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1641
If someone keeps a vow that he will fast on an assigned day, in the event that he intentionally invalidates his fast on that day, he must give kaffārah. The kaffārah for this will be mentioned in the laws relating to vows.