Sahih Muslim Book 4, Hadith Number 802.

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.

This Hadith has been transmitted by Qatida with the same chain of transmitters (and the words are): “Allah, the Exalted and the Glorious, commanded it through the tongue of His Apostle (may peace be upon-him): Allah listens to him who praises Him.”