Sahih Al Bukhari Hadith Number 740

Narrated Sahl bin Sa`d: The people were ordered to place the right hand on the left forearm in the prayer. Abu Hazim said, “I knew that the order was from the Prophet (peace be upon him) .”

10  Call to Prayers (Adhaan)
Hadith number 740
Volume no 1
Chapter: 87 (To place the right hand on the left [in As-Salt (the prayers)]