Musnad Imam Shafi'i

It is narrated from Abdullah bin Umar (may Allah be pleased with them both) that he said: "I saw the Messenger of Allah (ﷺ) when he began the prayer, he would raise his hands until they were level with his shoulders, and when he intended to bow (ruku‘), and when he raised his head from bowing, (he would also raise his hands), and he would not raise (his hands) between the prostrations."