Due to hardware limitations, the phase shifts of the reflecting elements of reconfigurable intelligent surfaces (RISs) need to be quantized into discrete values. This letter aims to unveil the minimum required number of phase quantization levels L in order to achieve the full diversity order in RIS-assisted wireless communication systems. With the aid of an upper bound of the outage probability, we first prove that the full diversity order is achievable provided that L is not less than three. If L=2, on the other hand, we prove that the achievable diversity order cannot exceed (N+1)/2, where N is the number of reflecting elements. This is obtained with the aid of a lower bound of the outage probability. Therefore, we prove that the minimum required value of L for achieving the full diversity order is L=3. Simulation results verify the theoretical analysis and the impact of phase quantization levels on RIS-assisted communication systems.