When to Stand for the Fajr Prayer
Question
(1) How long before the end of the time of Fajr should the congregational prayer be completed? (2) What is [the ruling on] standing for the Fajr prayer at a time when, after one rak'a or before the closing salam, [the time for ada' passes and] the time of qada' sets in?
Answer
(1) Early enough that, if on finishing the prayer it became known that it needs to be prayed again because for some reason the prayer had been spoiled, it could be prayed again in accordance with the sunna before sunrise. (2) Through this the prayer will become fasid (invalid); delaying [it] that long is not permissible — it is a sin.
Source: Fatawa Mahmudiyya
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