I was reading this...
PARABLE 0120: THIS MIND IS THE BUDDHA
"Once a monk asked Big Plum what [the famous Zen Patriarch] Matsu taught him.
Big Plum said, 'This mind is the Buddha.'
The monk replied, 'Nowadays Matsu teaches That which isn't the mind isn't the Buddha.'
To this Big Plum replied, 'Let him have That which isn't the mind isn't the Buddha. I'll stick with "This mind is the Buddha."'
When he heard this story, Matsu said, 'The plum is ripe.'
(Transmission of the Lamp, chapter 7)."Red Pine: 116