The sunnah is to grasp the knees.
أخبرنا سويد بن نصر قال أنبأنا عبد الله عن سفيان عن أبي حصين عن أبي عبد الرحمن السلمي قال
إنما السنة الأخذ بالركب
No references are available.
Sahih. Tirmidhi transmitted it in the Book of Prayer, chapter on placing the hands on the knees during ruku, hadith 258, through another route from Abu Husayn, with the same wording, and he said, hasan sahih. It is also in al-Kubra, hadith 624. The hadith has many corroborating narrations.