Ruling on leaving off standing in the obligatory prayer while able to do so

Question:
May Allah guide you, Shaykh. A questioner says: A woman prayed the obligatory prayer sitting down, even though she was able to stand. She was informed that this invalidates the prayer, but she was unaware of the ruling. However, she did not repeat it. What is required of her?

Answer:
She must repeat the prayer, now that the Islamic ruling has been explained to her and she has been told that she was in error by leaving out a pillar of the prayer. She must repeat the prayer.