What is the “difficulty” which makes it permissible to pray sitting down?

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Check below answers in case you are looking for other related questions:
When is it permissible for a person to pray sitting in obligatory prayers?
Rulings and issues about praying on a chair

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Related Answers:
Praying on rugs
Fardh and sunnah prayers must be performed separately
If the doorbell rings whilst I am praying
When is it permissible for a person to pray sitting in obligatory prayers?
Rulings and issues about praying on a chair

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