Muwatta Malik — Hadith 167

Yahya related to me from Malik from Ibn Shihab from Salim ibn Abdullah that Abdullah ibn Umar used to say "Allah is greater" in the prayer whenever he lowered himself and raised himself. Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to raise his hands to the level of his shoulders when he began the prayer and when he raised his head from the ruku he would raise them less than that.