Sahih Ibn Khuzaymah

Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) narrates from the Noble Prophet (peace and blessings be upon him) that when the Prophet (peace and blessings be upon him) would stand for the obligatory prayer, he would say « اللهُ أَكْبَرُ » and raise both his hands up to his shoulders. When he completed his recitation and intended to bow (ruku‘), he would also perform the raising of the hands in the same manner, and when he raised his head from ruku‘, he would again raise his hands in the same way. And the Prophet (peace and blessings be upon him) would not raise his hands while sitting in his prayer. And when he stood up after the tashahhud of two rak‘ahs, he would raise his hands in the same way and say « اللهُ أَكْبَرُ ».