From Sahih Muslim (Hadith 1095):
Ibn 'Abbas reported:The Apostle of Allah (ﷺ) had been commanded that he should prostrate on the seven (bones) and he was forbidden to fold back the hair and clothing. And in the narration transmitted by Abu Rabi' (the words are):" on the seven bones and I was forbidden to fold back the hair and clothing". According to Abu'l-Rabi' (the seven bones are): The hands, the knees, and the (extremities) of the feet and the forehead
Reference: Muslim 1095