ANSWER:

If you remembered soon after salam, stand up and offer the missed rakah and offer 2 prostrations after salam for forgetfulness. 

But if you remembered this long after the pray was over, your prayer is invalid and you must offer it again. 

What should a person do if he joins the prayer when the imaam has alredy finished one rakat and at the end the person forgets that he had missed one rakat and finishes the salah with the imaam, does he have to repeat the whole salah or not?

Prayer | June 21, 2020 4:29 am
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In my country fajr start around 3.34 AM & dhuhr 11.56AM (I took this time from internet) . But the adhan for fajr is given on around 4 AM & dhuhr at 1.20PM. To pray salah should I wait for the adhan to be called or I can perform salah as the prescribed time begin?
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I know that we are not supposed to be praying the sunnah for before fajr while the fard has begun in the masjid. Does the same ruling apply for when we’re praying at home? If I am praying sunnah and my family starts fard, should I stop?