Fasting: Invalidating a Fast After Investigating the Time of Subh
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1661
If in the month of Ramadan, after investigating (by looking at the horizon to sight the true dawn),[1] it is not known to someone that the time of ṣubḥ prayers has set in, and he does something that invalidates a fast, and afterwards it becomes known that it was ṣubḥ, then qaḍāʾ is not necessary.
-Ayatullah Sistani, Practical Laws of Islam, Fasting (Sawm)
1
See Ruling 728