Sahih Al Bukhari Hadith Number 735

Narrated Salim bin `Abdullah: My father said, “Allah’s Messenger (peace be upon him) used to raise both his hands up to the level of his shoulders when opening the prayer; and on saying the Takbir for bowing. And on raising his head from bowing he used to do the same and then say “Sami`a l-lahu liman hamidah, Rabbana wa laka l-hamd.” And he did not do that (i.e. raising his hands) in prostrations.

10  Call to Prayers (Adhaan)
Hadith number 735
Volume no 1
Chapter: 83 (To raise both hands on saying the first Takbir simultaneously with opening the Salat (prayer)