Sahih Bukhari
Characteristics of Prayer
Volume: 001 / Book Number: 012 / Hadith Number: 776
Narrated Ibn 'Abbas:
The Prophet said, "I have been ordered to prostrate on seven bones i.e. on the forehead along with the tip of the nose and the Prophet pointed towards his nose, both hands, both knees and the toes of both feet and not to gather the clothes or the hair."