I said prayer by the side of my father and placed my hands between my knees. My father said to me: Place your hands on your knees. I repeated that (the previous act) for the second time, and he struck at my hands and said: We have been forbidden to do so and have been commanded to place our palms on the knees.  

لِي أَبِي اضْرِبْ بِكَفَّيْكَ عَلَى رُكْبَتَيْكَ قَالَ ثُمَّ فَعَلْتُ ذَلِكَ مَرَّةً أُخْرَى فَضَرَبَ يَدَىَّ وَقَالَ إِنَّا نُهِينَا عَنْ هَذَا وَأُمِرْنَا أَنْ نَضْرِبَ بِالأَكُفِّ عَلَى الرُّكَبِ  

[Machine] O my son, hit your knees with your hands then I did it again after that by a day, and I prayed towards it and he slapped my hands. When he left, he said, "We used to do this and we were commanded to hit with the palms on the knees."  

يَا بُنَيَّ اضْرِبْ بِيَدَيْكَ رُكْبَتَيْكَ ثُمَّ فَعَلْتُهُ مَرَّةً أُخْرَى بَعْدَ ذَلِكَ بِيَوْمٍ فَصَلَّيْتُ إِلَى جَنْبِهِ فَضَرَبَ يَدِي فَلَمَّا انْصَرَفَ قَالَ «كُنَّا نَفْعَلُ هَذَا وَأُمِرْنَا أَنْ نَضْرِبَ بِالْأَكُفِّ عَلَى الرُّكَبِ»