عن ابن عباس رضي الله عنهما عن النبي صلى الله عليه وسلم قال: «أمِرْت أن أسْجُد على سَبْعَة أعَظُم على الجَبْهَة، وأشار بِيَده على أنْفِه واليَدَين والرُّكبَتَين، وأطْرَاف القَدَمين ولا نَكْفِتَ الثِّياب والشَّعر». [صحيح] - [متفق عليه] المزيــد ...
Ibn ‘Abbās (may Allah be pleased with him) reported: 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." Sahih/Authentic. - [Al-Bukhari and Muslim]
The Prophet (may Allah's peace and blessings be upon him) said: "I have been commanded to prostrate…” In another narration he said: "We have been commanded…” and a third narration says: "The Prophet (may Allah's peace and blessings be upon him) commanded...” All these are recorded in Al-Bukhāri. There is a basic Islamic principle that anything the Prophet (may Allah's peace and blessings be upon him) is commanded to observe is an instruction given to him and to his community at large. "On seven bones", meaning: On seven parts of the body, which another narration specifies as “body parts of prostration.” These are the forehead including the nose, the hands including the palms, the knees, and the extremities of the feet. In the Hadīth of Abu Humayd As-Sa‘īdi (may Allah be pleased with him): "He pointed his toes toward the Qiblah", meaning: while prostrating. "And not to tuck up the clothes and hair'', meaning: Do not prevent one's clothes and hair from spreading out when bowing and prostrating. They should be left to fall to the ground, so everything can prostrate to Allah: parts of the body, clothes, and hair.
It is obligatory to prostrate in prayer upon the seven limbs. A command basically signifies an obligation.
Prostrating upon the forehead without the nose is not valid, neither is prostration upon the nose is valid without the forehead. When the Prophet (may Allah’s peace and blessings be upon him) mentioned the forehead, he pointed to the nose.
It is obligatory to prostrate upon the whole limb, not only part of it. One should place as much of his forehead on the ground as he can.
Apparently, one is not required to reveal any of these limbs during prostration, for the need to prostrate upon them is fulfilled by placing them on the ground without revealing them. Revealing the knees is undisputedly not obligatory, given the fear of showing ‘Awrah, and so is revealing the feet, given the permissibility of praying while wearing leather socks.
It is disliked to fold back one’s clothing during prayer.
It is disliked to braid one’s hair and put it behind his nape, whether he does it intentionally for prayer or for any other purpose and then enters prayer in this condition without necessity.