How to Pray Sitting on a Chair
If a person offers an obligatory prayer sitting when he is able to stand, his prayer is invalid. Whoever is not able to stand, bow, and prostrate because of sickness or old age, it is Sunnah for him to sit on the ground or on a chair.
If the worshipper is going to pray on the chair from the beginning of the prayer until the end, then he should make the place where he will sit level with the row.
If he is going to pray standing, but he will sit on the chair when he bows and prostrates, then what matters here is the standing position, so he should be level with the row when standing.
186,185