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.  

عَنِ ابْنِ عَبَّاسٍ أُمِرَ النَّبِيُّ ﷺ أَنْ يَسْجُدَ عَلَى سَبْعَةِ أَعْضَاءٍ وَلاَ يَكُفَّ شَعَرًا وَلاَ ثَوْبًا الْجَبْهَةِ وَالْيَدَيْنِ وَالرُّكْبَتَيْنِ وَالرِّجْلَيْنِ  

The Prophet ﷺ said, "We have been ordered to prostrates on seven bones and not to tuck up the clothes or hair."  

عَنِ النَّبِيِّ ﷺ قَالَ أُمِرْنَا أَنْ نَسْجُدَ عَلَى سَبْعَةِ أَعْظُمٍ وَلاَ نَكُفَّ ثَوْبًا وَلاَ شَعَرًا  

(He was not a liar) We used to pray behind the Prophet ﷺ and when he said, "Samiʿa l-lahu liman hamidah", none of us would bend his back (to go for prostration) till the Prophet ﷺ had placed his, forehead on the ground.  

كُنَّا نُصَلِّي خَلْفَ النَّبِيِّ ﷺ فَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ لَمْ يَحْنِ أَحَدٌ مِنَّا ظَهْرَهُ حَتَّى يَضَعَ النَّبِيُّ ﷺ جَبْهَتَهُ عَلَى الأَرْضِ