I have been commanded, and sometimes the narrator said: Your prophet (ﷺ) was commanded to prostrate on seven limbs.
Sunan Abi Dawud Book 2, Hadith 500
← Previous Hadith Ibn Abbas reported the Prophet (ﷺ) as saying: I have been commanded - according to the version of Hammad: Your Prophet (ﷺ) was commanded - to prostrate on seven (bones), and not to fold back the hair or the clothing.

Sunan Abi Dawud Book 2, Hadith 499 Next Hadith → when a servant (of Allah) prostrates himself, the seven limbs, i.e, his face, his palms, his knees and his feet prostrate along with him.

Sunan Abi Dawud Book 2, Hadith 501