Musnad Ahmed: Hadith 3949

The Messenger of Allah (ﷺ) taught us the prayer: he said takbeer and raised his hands, then he bowed and put his hands together and placed therm between his knees. News of that reached Sa’d and he said: My brother has spoken the truth; we used to that, then we were commanded to do this, and he took hold of his knees (with his hands).