Sahih Al Bukhari Hadith Number 736

Narrated `Abdullah bin `Umar: I saw that whenever Allah’s Messenger (peace be upon him) stood for the prayer, he used to raise both his hands up to the shoulders, and used to do the same on saying the Takbir for bowing and on raising his head from it and used to say, “Sami`a l-lahu liman hamidah”. But he did not do that (i.e. raising his hands) in prostrations.

10  Call to Prayers (Adhaan)
Hadith number 736
Volume no 1
Chapter: 84 (To raise both hands while saying the Takbir [on opening As-Salat (the prayer)], and while bowing and on raising up the head (after bowing)