Does prayer become obligatory immediately upon waking during the period of discrepancy in Fajr prayer times between the two mosques, and is it permissible to eat and drink with the intention of fasting during this period?

The time for Fajr prayer begins with the appearance of the true dawn (al-Fajr al-Sadiq). It may be difficult for city dwellers to ascertain its precise start due to artificial lights. However, the prayer can be delayed until one is certain that the time has begun or has a strong presumption (ghalabat al-dhann). Jurists have stipulated that prayer is not performed until one has certain knowledge of the time's entry or a strong presumption, as mentioned by Ibn Qudamah in "Al-Mughni." Similarly, abstaining from food and drink during fasting (imsak) occurs upon certainty or a strong presumption of the dawn's arrival, though abstaining out of caution when in doubt is preferable.