What Buffer register saves the instruction?

It is employed to hold temporarily the right hand instruction from a Word in memory..

For example, The IAS machine's basic unit of information was a 40-bit, so that two instructions could be stored in each 40-bit memory location. Each instruction consisted of an 8-bit {operation code} and a 12-bit address.

Hence the IBR (Instruction Buffer Register) is used to temporarily hold Right hand instruction for the next use.