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.

-Ayatullah Sistani, Practical Laws of Islam, Fasting (Sawm)