Is it allowed to pray salaat in jammat with other people for a namaaz that has been qadha ie can we pray fajr in jammat at zuhr time and does it have the same reward as normal jamaat prayer?

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You can’t pray your qadha fajr namaz with Jamat of Zohr namaz if you have not prayed Zohr already.
Yes if your Zohr is Qasr and you finished your 2 Rakat Qasr namaz with Zohr Jamat namaz then in 3rd Rakat of Zohr with Imam you can pray your 2 Rakats of Qadha of fajr namaz But from beginning you can’t start your fajr qadha namaz with Jamat of Zohr namaz.