Sahih Muslim Book 4, Hadith Number 1092.

Chapter : The virtue of building the mosques and exhortation to it.

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.