(2) باب الإِمْسَاكِ بِالرُّكَبِ فِي الرُّكُوعِ
It was narrated that 'Abdur-Rahman As-Sulami said:
" 'Umar said: 'The Sunnah is to hold the knees.'"
Reference: Sunan an-Nasa'i 1035
In-book reference: Book 12, Hadith 7
English translation: Vol. 2, Book 12, Hadith 1036