Sahih Muslim Book 4, Hadith Number 797.

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.

Ibn Mas’ud is reported to have said: The Messenger of Allah (may peace be upon him) taught me tashahhud taking my hand within his palms, in the same way as he taught me a Sura of the Qur’an, and he narrated it as narrated above.