Sahih Muslim Book 4, Hadith Number 799.

Chapter : The placing of the right hand over the left hand after the first Takbir in prayer (Takbir-i-Tahrima) below the chest and above the navel and then placing them opposite the shoulders in prostration.

Tawus narrated it on the authority of Ibn ‘Abbas that he said: The Messenger of Allah (may peace be upon him) used to teach us tashahhud as he would teach us a Sura of the Qur’an.