Fasting: Intentionally Invalidating a Fast When Doubting Between Ramadan and Shawwāl
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1646
If a person doubts whether it is the last day of the month of Ramadan or the first of Shawwāl and he intentionally invalidates his fast, and afterwards it becomes known that it was the first of Shawwāl, kaffārah is not obligatory for him.