Sahih Muslim Book 4, Hadith Number 795.

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 narrated on the authority of Mansur with the same chain of transmitters and he made a mention of this: “Then he may choose any supplication which pleases him or which he likes.”