It is narrated from Abu Abd al-Rahman al-Sulami (may Allah have mercy on him) that Sayyiduna Umar (may Allah be pleased with him) came forward and said: "O people! The sunnah way for you is that you hold the knees, so hold the knees in bowing."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2546