My question is if I wake up the usual time for Sehri before Fajr, and then I realize that I need to do ghusl al-Jannaba, but instead I do a quick Sehri, and go to do the ghusl 4-5 mins before the Adhan of Fajr, and when I come out of the bathroom the Fajr Adhan has already take place, so is this fast valid?

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If dawn started before finishing the ghusl, then you need to re make this day.