It is narrated from Mus'ab bin Sa'd (may Allah have mercy on him) that he said: I was praying beside my father. When I bowed, I did like this. He joined his hands together and placed them between his legs. When my father finished the prayer, he said: "We too used to do this, but later we were commanded to place the hands on the knees."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2545