He heard the Messenger of Allah ﷺ say: "When a person prostrates, seven parts of his body prostrate: his face, his two palms, his two knees and his two feet."  

أَنَّهُ سَمِعَ رَسُولَ اللَّهِ ﷺ يَقُولُ إِذَا سَجَدَ الْعَبْدُ سَجَدَ مِنْهُ سَبْعَةُ آرَابٍ وَجْهُهُ وَكَفَّاهُ وَرُكْبَتَاهُ وَقَدَمَاهُ