Rabi' bin Sulayman (may Allah have mercy on him) said: I asked Imam al-Shafi'i (may Allah have mercy on him): What is the meaning of raising the hands at the time of bowing? He replied: The same meaning that raising them at the beginning of the prayer has is what is intended by it.
Then he said: In it is an expression of the greatness of Allah and the following of the Sunnah of the Messenger of Allah (peace and blessings be upon him), and there is also the hope of reward from Allah; and it is like raising the hands on Safa and Marwah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2537