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.‏