The hands prostrate as the face prostrates, so when one of you puts his face down he should put his hands down, and when he raises (the face) he should raise (the hands) too. (Using translation from Nasāʾī 1092)
"إِنَّ اليَدَيْن يسْجُدان كما يسْجُد الوجْهُ، فَإذَا وضَع أحدُكُمْ وجْهَهُ فليضَعْ يديْهِ، وِإذَا رفَعهُ فَليرْفَعْهُمَا ".