Sahih Muslim Book 4, Hadith Number 798.

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 ‘Abbas reported: The Messenger of Allah (may peace be upon him) used to teach us tashahbud just as he used to teach us a Sura of the Qur’an, and he would say: All services rendered by words, acts of worship and all good things are due to Allah. Peace be upon you, O Prophet and Allah’s mercy and blessings. Peace be upon us and upon Allah’s upright servants. I testify that there is no god but Allah, and I testify that Muhammad is the Messenger of Allah. In the narration of Ibn Rumb (the words are): “As he would teach us the Qur’an.”