The time for suhoor is until dawn break. As soon as dawn breaks, the time for suhoor ends and simultaneously, the time for fajr begins. In essence, the end of the time for suhoor depends on the actual time, not the azan, because the azan is called at different times in different mosques. Thus, it is necessary to stop eating and drinking as soon as the time for suhoor ends. Consequently, eating and drinking after or even during the azan is not permissible. Whoever continues to eat or drink after the time for fajr has begun, whether knowingly or unknowingly, in both cases such a person invalidates their fast and is required to perform qaza for it later.
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دلائل:
الھندیة: (194/1، ط: رشیدیة)
تسحر علی ظن أن الفجر لم یطلع وہو طالع أو أفطر علی ظن أن الشمس قد غربت ولم تغرب قضاہ ولا کفارۃ علیہ؛ لأنہ ما تعمد الإفطار۔
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.