Is it a condition for sitting in the obligatory prayer to be unable to stand only, or does that include being able to do so with difficulty and pain?

Standing in the obligatory prayer is a pillar that may not be abandoned except in cases of inability or excessive hardship that is usually difficult to bear, as the Prophet, peace and blessings be upon him, said: "Pray standing; if you are unable, then sitting; if you are unable, then on your side."

Shaykh Ibn Uthaymin clarified that the criterion for the hardship that permits sitting in prayer is that which removes khushu' (devotional humility), presence of heart, and tranquility.