«كان يضع اليمنى على اليسرى في الصلاة............ ... »
It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would place his right hand over his left hand in the prayer."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4975