When a worshipper prostrates, he must prostrate on all seven parts of the body on which the Prophet (peace and blessings of Allaah be upon him) commanded us to prostrate. They are: the face, including the forehead and nose; the two hands; the two knees; and the edges of the two feet (i.e., the toes).
Al-Nawawi said: If he omits any of them, then his prayer is not valid. From Sharh Muslim.
Shaykh Ibn ‘Uthaymeen said:
It is not permissible for a worshipper to lift up any of these seven parts of the body when prostrating, because the Prophet (peace and blessings of Allaah be upon him) said: “I have been commanded to prostrate on seven bones: on the forehead,” and he pointed to his nose, “and on the two hands, the two knees and the edges of the two feet (i.e., the toes).” Narrated by al-Bukhaari, 812; Muslim, 490. If he lifts up one or both of his feet, or one or both of his hands, or his forehead or nose, or both of them, then his prostration is invalid and does not count, and if his prostration is invalid then his prayer is also invalid.
Liqaa’aat al-Baab al-Maftooh by Shaykh Ibn ‘Uthaymeen, 2/99.