Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(133)
Chapter: To prostrate on seven bones
(133)
باب السُّجُودِ عَلَى سَبْعَةِ أَعْظُمٍ
Sahih al-Bukhari 809

Narrated Ibn `Abbas:

The Prophet (ﷺ) 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.

حَدَّثَنَا قَبِيصَةُ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَمْرِو بْنِ دِينَارٍ، عَنْ طَاوُسٍ، عَنِ ابْنِ عَبَّاسٍ، أُمِرَ النَّبِيُّ صلى الله عليه وسلم أَنْ يَسْجُدَ عَلَى سَبْعَةِ أَعْضَاءٍ، وَلاَ يَكُفَّ شَعَرًا وَلاَ ثَوْبًا الْجَبْهَةِ وَالْيَدَيْنِ وَالرُّكْبَتَيْنِ وَالرِّجْلَيْنِ‏.‏
Reference : Sahih al-Bukhari 809
In-book reference : Book 10, Hadith 204
USC-MSA web (English) reference : Vol. 1, Book 12, Hadith 773
  (deprecated numbering scheme)