Sayyiduna Ibn Umar (may Allah be pleased with them both) said: "When the Messenger of Allah (peace and blessings be upon him) began the prayer, he would raise both his hands, and also when saying the takbir for bowing (ruku'), and also when raising his head from bowing."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13174