What should a Muslim do to be under Allah's protection if they miss the Fajr prayer, and how true is it that missing the congregational prayer is due to a sin?

Whoever prays Fajr is under the protection of Allah Almighty. This means that the hadith (definition) warns against harming anyone who has prayed Fajr. It is also said that the intention is His preservation and care for them. The soul is fortified by the morning and evening remembrances (adhkar), by frequently supplicating, remembering Allah Almighty, and having piety, as stated in the hadith: "Guard Allah, and He will guard you." Some of the predecessors are reported to have said: "No one missed congregational prayer except due to a sin they committed."