All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
The act of the Sunnah, in this case, is to place your hands on your knees when performing bowing and on the floor when prostrating, if that is possible; otherwise, place them on your knees. Shaykh Ibn Baaz wrote:
"The one who prays sitting on the ground or on a chair must make his Sujood (prostration) lower than his Rukoo‘ (bowing). The Sunnah is for him to put his hands on his knees in the Rukoo‘ and to put them on the ground in Sujood if possible; if he cannot do that, then he puts them on his knees, because it was proven that the Prophet, sallallahu ‘alayhi wa sallam, said, 'I have been commanded to prostrate on seven bones: the forehead – and he, sallallahu ‘alayhi wa sallam, pointed to his nose – the two hands, the two knees, and the toes of both feet.'" [Majmoo‘ Al-Fataawa]
Allah knows best.