The Messenger of Allah ﷺ would bow by placing his hands on his knees and holding his upper arms away from his sides.
حدثنا أبو بكر بن أبي شيبة حدثنا عبدة بن سليمان عن حارثة بن أبي الرجال عن عمرة
كان رسول الله ﷺ يركع فيضع يديه على ركبتيه ويجافي بعضديه
[Hasan] See hadith no. 56 regarding its flaw, and the hadith has hasan corroborating narrations in Abu Dawud and elsewhere.