Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 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.