The World Federation One Stop Fiqh

Ruling 1661

If in the month of Ramadan, after investigating (by looking at the horizon to sight the true dawn),10 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.

10See Ruling 728.