Sahih Ibn Khuzaymah

Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates that when the Messenger of Allah (peace and blessings be upon him) would stand for prayer, he would raise both his hands up to his shoulders, then say « اللهُ أَكْبَرُ ». Then, when he intended to bow (ruku‘), he would do the same (raise the hands), and when he raised his head from bowing, he would do the same (raise the hands). And when he raised his blessed head from prostration, he would not raise his hands.