Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.124A.

These parts are: the face, hands, knees and feet. Al-‘Abbas ibn ‘Abdul-Mutallib reported that he heard the Prophet say, “When a slave (of Allah) prostrates, seven bodily parts prostrate with him: his face, his hands, his knees and his feet.” (Related by “the group,” except for al-Bukhari.) Said Ibn ‘Abbas, “The Prophet ordered us to prostrate on seven bodily parts and not to fold back the hair or clothing: the forehead, the hands, the knees and the feet.” In another wording, the Prophet said, “I have been ordered to prostrate on seven bodily parts: the forehead, and he pointed to his nose, the hands, the knees and the ends of the feet.” (Related by al-Bukhari and Muslim.) In another narration, he said, “I have been ordered to prostrate on seven bodily parts and not to fold back the hair or clothing: the forehead, the nose, the hands, the knees and the feet.” (Related by Muslim and an-Nasa’i.)

Abu Humaid reported that when the Prophet, upon whom be peace, prostrated, he placed his nose and forehead on the ground. This hadith is related by Abu Dawud and at-Tirmizhi who said, “The scholars act according to this: a person prostrates on his nose and forehead.” According to some scholars, if one prostrates on just the forehead without the nose touching the ground, it will still be sufficient. Others say that it would not be sufficient until his nose touches the ground.