In this letter, we jointly optimize transmitter source currents and receive coil selection in a magnetic multiple input multiple output wireless power transfer system to minimize the total transmit power, while satisfying a required level of the charged power and the limited total transmit power. The optimal transmitter source currents are derived for an arbitrary number of receive coils. Based on our analysis, we propose a method to find the optimal subset of the receive coils. Our simulation results show that the proposed scheme requires the lowest transmit power to meet the constraint on the charged power.