Explanation of that
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا بَكْرٌ، عَنِ ابْنِ الْهَادِ، عَنْ مُحَمَّدِ بْنِ إِبْرَاهِيمَ، عَنْ عَامِرِ بْنِ سَعْدٍ، عَنِ الْعَبَّاسِ بْنِ عَبْدِ الْمُطَّلِبِ، أَنَّهُ سَمِعَ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " إِذَا سَجَدَ الْعَبْدُ سَجَدَ مِنْهُ سَبْعَةُ آرَابٍ وَجْهُهُ وَكَفَّاهُ وَرُكْبَتَاهُ وَقَدَمَاهُ " .
It was narrated from Al 'Abbas bin 'Abdul-Muttalib that : 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."