The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1092
Mus'ab b. Sa'd b. Abu Waqqas reported: I said prayer by the side of my father. When I bowed I intertwined my fingers and placed them between my knees. He struck my hands. When he completed the prayer he said: We used to do that but then were commanded to lift (our palms) to the knees.