If the jamah has started, you have only one option and that is to pray it after the jamah is over or aft sunrise.

It is forbidden to pray it while the jamah is on

In Fajr prayer if we see jamaa’ah has started then what is the best option for praying the 2 units Sunnah prayer of Fajr?

Prayer | March 25, 2014 4:14 am
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I read in the description of the prophets prayer in this website that it is allowed to pray wearing shoes then how about the purity of the shoes because we walk on many impure things?
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I live in the UK where isha prayer in the summer becomes very late (reaches 11:40pm) and fajr 3am, so my local mosque is combining maghrib and isha prayer. But the other mosque 5 minutes away from it doesn’t combine (they are both shafi’i madhhab). I have been told by the imam that (combines maghrib and isha) this is permissible for everyone, even if you do not have work in the morning or something. Can you please tell me the rulings on this because I am confused,