It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that when the Messenger of Allah (peace and blessings be upon him) said the takbir in the prayer, he would raise both his hands to the level of his shoulders; when he bowed, he would do the same; when he raised his head, he would do the same; and when he said "Sami'a Allahu liman hamidah" (Allah hears the one who praises Him), he would say "Rabbana laka al-hamd" (Our Lord, to You belongs all praise) and do the same; and he would not do so in the prostration.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13056