"It is established for you to hold the knees, so hold the knees."  

سُنَّتْ لَكُمُ الرُّكَبُ فَأَمْسِكُوا بِالرُّكَبِ  

" 'Umar said: 'The Sunnah is to hold the knees.'"  

عُمَرُ إِنَّمَا السُّنَّةُ الأَخْذُ بِالرُّكَبِ