“I have been commanded to prostrate upon seven bones: the forehead"—and he pointed with his hand to his nose—"the two hands, the two knees, and the tips of the toes; and not to fold up the garments or gather the hair.” 

Narrated by al-Bukhārī (no. 812) and Muslim (no. 490), from the narration of Ibn ʿAbbās (may Allah be pleased with them both).

Brief Explanation of the Hadith

Prayer holds immense status and profound importance. It has conditions, pillars, obligations, and recommended acts. Among its greatest pillars is prostration (sujūd). The Prophet (may Allah’s peace and blessings be upon him) clarified its proper manner, saying: “I have been commanded to prostrate upon seven bones,” i.e., seven bodily parts. Meaning: I have been commanded to place these seven parts of the body on the ground during prostration. The command here indicates obligation; thus, the prayer is not valid without prostrating upon them—except in the case of a valid excuse.
Then the Prophet (may Allah’s peace and blessings be upon him) clarified them, saying: “The forehead”—and he pointed with his hand to his nose—to indicate that they are considered one part, as the bone of the nose branches from the bone of the forehead.
He then specified the hands, the knees, and the tips of the feet. These make seven body parts: the forehead and nose count as one, the two hands (i.e., the palms and fingers) are two, the two knees are two, and the toes of both feet are two—making a total of seven parts.
The scholars have differed regarding prostration upon the folded portion of a turban (kawr al-ʿimāmah). However, it is preferable to place the forehead directly on the ground, especially when the turban is thick.
Then the Prophet (may Allah’s peace and blessings be upon him) forbade the gathering (kaft) of garments and hair.
Kaft of garments refers to folding or bunching up parts of one’s clothing during bowing or prostration, and kaft of hair is an expression referring to tying or bundling it to prevent it from touching the ground in sujūd, as a way of shielding it from dust.
According to the majority of scholars, this prohibition is understood to indicate dislikedness (karāhah).
It has been said that the wisdom behind the prohibition is that if one lifts his garment or hair to prevent it from touching the ground, it resembles the posture of the arrogant, or because the hair prostrates along with him.
However, this prohibition does not apply to one who gathers his garment out of fear that his ʿawrah might become exposed—for that is in fulfilment of an obligation.
The prohibition against gathering or tying up the hair (kaft al-shaʿr) applies specifically to men. As for women, they are not included in this ruling, because undoing their braids entails hardship and and it alters their appearance in a way that goes against the natural sense of beauty.
According to the majority of scholars, the prohibition against tying the hair or gathering the garments (ʿaqṣ al-shaʿr and kaft al-thiyāb) is general for anyone performing prayer—whether done during the prayer or beforehand, even if not done specifically for the prayer.