Sahih Al Bukhari Hadith Number 810

Narrated Ibn `Abbas: The Prophet (peace be upon him) was ordered (by Allah) to prostrate on seven parts and not to tuck up the clothes or hair (while praying). Those parts are: the forehead (along with the tip of nose), both hands, both knees, and (toes of) both feet.

10  Call to Prayers (Adhaan)
Hadith number 810
Volume no 1
Chapter: 133 (To prostrate on seven bones)