Sayyiduna Ibn Umar (may Allah be pleased with them both) said: "I saw the Messenger of Allah (peace and blessings be upon him) say the takbir in the prayer and raise both his hands up to the level of his shoulders, then when he said the takbir for bowing, he did the same." Then he (peace and blessings be upon him) said "Sami'a Allahu liman hamidah" (Allah hears the one who praises Him) and did the same, then he said "Rabbana laka al-hamd" (Our Lord, to You belongs all praise) and did the same.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13055