Not-Buddhism is the highest level of Buddhism?

I'm new here. I have a question regarding "Not-Buddhism" as the highest level of Buddhism. I'm not sure where is the source, but this is the koan. >Tozan said to his monks, "You monks should know there is an even higher understanding in Buddhism." A monk stepped forward and asked, "What is the higher Buddhism?" Tozan answered, "**It is not Buddha.**" What does Tozan mean by "It is not Buddha"?