Sahih Bukhari
Characteristics of Prayer
Volume: 001 / Book Number: 012 / Hadith Number: 756
Narrated Mus'ab bin Sa'd:
I offered prayer beside my father and approximated both my hands and placed them in between the knees. My father told me not to do so and said, "We used to do the same but we were forbidden (by the Prophet) to do it and were ordered to place the hands on the knees."