What is the ruling on one who prays some of the rak'ahs standing and some of them sitting due to fatigue or knee pain?

If one is unable to stand in prayer, or experiences clear hardship in doing so, it is permissible for him to pray sitting. His prayer is valid, and his reward will not be diminished, due to the Prophet's (peace be upon him) saying: "Pray standing; if you are unable, then sitting; and if you are unable, then on your side." And also his saying: "When a servant falls ill or travels, what he used to do when healthy and résident will be recorded for him." So, if a woman prayed sitting in a situation where it was permissible for her to sit, her prayer is valid. However, if she was negligent and prayed sitting due to a hardship that is not recognized (as valid), then she must make up the prayer, as the pillar of standing was missed.