Hadith: I have been commanded to prostrate on seven bones

عن ابنِ عَبَّاسٍ رضي الله عنهما عن النبيِّ صلى الله عليه وسلم قال:
«أُمِرْتُ أَنْ أَسْجُدَ عَلَى سَبْعَةِ أَعْظُمٍ: عَلَى الْجَبْهَةِ وَأَشَارَ بِيَدِهِ عَلَى أَنْفِهِ، وَالْيَدَيْنِ، وَالرُّكْبَتَيْنِ، وَأَطْرَافِ الْقَدَمَيْنِ، وَلَا نَكْفِتَ الثِّيَابَ وَالشَّعَرَ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 812]

Ibn ‘Abbās (may Allah be pleased with him and his father) reported that the Prophet (may Allah's peace and blessings be upon him) said:
"I have been commanded to prostrate on seven bones: the forehead (and he pointed to his nose), the hands, the knees, and the extremities of the feet, and not to tuck up the clothes and hair."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 812]

Explanation

The Prophet (may Allah's peace and blessings be upon him) clarified that Allah commanded him to prostrate in prayer on seven body parts, which are:
First: The forehead, i.e., the upper part of the face above the nose and eyes. The Prophet (may Allah's peace and blessings be upon him) pointed with his hand to his nose to clarify that the forehead and the nose represent one part of the seven and to stress the fact that one must touch the ground with his nose in prostration.
The second and third parts: The two hands.
The fourth and fifth: The knees.
The sixth and seventh: The toes of both feet.
He commanded us not to tie our hair or fold up our clothes when prostrating on the ground to protect them; rather, we should let them spread out and fall on the ground to prostrate along with the body parts.