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. (Using translation from Abū Dāʾūd 891)
أَنَّهُ سَمِعَ رَسُولَ اللهِ ﷺ يَقُولُ إِذَا سَجَدَ الْعَبْدُ سَجَدَ مَعَهُ سَبْعَةُ آرَابٍ وَجْهُهُ وَكَفَّاهُ وَرُكْبَتَاهُ وَقَدَمَاهُ