It is narrated from Abu Ya'fur (may Allah have mercy on him) that he said: I heard Mus'ab bin Sa'd (may Allah have mercy on him) say: I prayed beside my father, and I joined my palms together and placed them between my thighs. My father (may Allah be pleased with him) forbade me from that and said: "We too used to do this before; then we were forbidden from it and were commanded to place our hands on the knees."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2543