Sahih Muslim Book 4, Hadith Number 1089.

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

Mus’ab b. Sa’d reported: I said prayer by the side of my father and placed my hands between my knees. My father said to me: Place your hands on your knees. I repeated that (the previous act) for the second time, and he struck at my hands and said: We have been forbidden to do so and have been commanded to place our palms on the knees.