Where should a person who is praying on a chair due to an excuse place his hands during prostration?

He who prays sitting on a chair, if he is able to prostrate on the seven limbs, it is obligatory for him to do so. If he is unable to do so, then prostrating on the ground is waived for him, and he should gesture. And if he gestures for prostration, he should place his hands on his knees. If he is able to place them on the ground, he should do so.