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)
قَالَ رَسُولُ اللهِ ﷺ إِذَا سَجَدَ ابْنُ آدَمَ سَجَدَ مَعَهُ سَبْعَةُ آرَابٍ وَجْهِهِ وَكَفَّيْهِ وَرُكْبَتَيْهِ وَقَدَمَيْهِ