Mus'ab bin Sa'd narrated that when the Banu Abdullah bin Mas'ud would bow (in ruku'), they would place their hands between their thighs. So I prayed next to (my father) Sa'd (bin Abi Waqqas) and did the same, but he struck my hand. When the prayer was finished, he said: "Son, place your hands on your knees (in ruku')." The next day, I again prayed next to him and did the same, and again he struck my hand. When the prayer was finished, he said: "We used to place (our hands joined) between our thighs like this, but we were commanded to place our palms on our knees."