Reference: Sharh ‘Umdatil-Fiqh – Tape No.10, Question No.50
Question: Does the day begin with the arrival of [the time of] Fajr, or the rising of the sun?
Response: [The day] begins with the arrival of [the time of] Fajr; fasting and the Fajr prayer begin with the arrival of [the time of] Fajr.
