Dogen's Dharma Hall Discourse, No. 319:

The true Dharma correctly transmitted by buddhas and ancestors is simply just sitting. My late teacher Tiantong [Rujing] instructed the assembly saying, "You know the circumstances of Zen Master Damei Fachang when he studied under the great teacher Kiangsi Mazu, don't you? He asked Mazu, 'What is Buddha?' and Mazu said, 'Mind itself is Buddha.' Immediately Damei made prostrations and departed, and entered the peaks of Plum Mountain (Meishan) where he ate pine nuts and wore lotus leaves, spending his whole life practicing zazen day and night. For thirty years he was unknown by rulers or ministers, and did not receive any support of patrons. This is an excellent example of the Buddha way."

We should know that zazen is the decorous activity of practice after realization. Realization is simply just sitting zazen. At this monastery we have the first monks' hall, so in this country of Japan this is the first we have heard of this, the first time we have seen it, the first time we have entered it, and the first time of sitting in the monks' hall. This is fortunate for people studying the Buddha way.

Later a monk asked Damei, "What principle did you attain when you saw great teacher Mazu and came to reside on this mountain?"
Damei said, "Mazu told me that this mind itself is Buddha."
The monk said, "These days Mazu's Buddha Dharma is different."
Damei said, "How is it different?"
The monk said, "These days he says, 'Neither mind nor Buddha.'"
Damei said, "This old man confuses people endlessly. I will let him have neither mind nor Buddha; for me it's just this mind itself is Buddha."
The monk returned and reported this to Mazu, and Mazu said, "The plum has ripened."

So we see that the person who clearly understood that this mind itself is Buddha abandoned human society and entered the deep mountains and valleys, only practicing zazen day and night. Brothers on this mountain, you should straightforwardly, single-mindedly focus on zazen. Do not pass your days and night in vain. Human life is impermanent; how can we wait for some other time? I prayerfully beseech your practice. Great assembly, do you want to understand the truth of this mind itself is Buddha?

After a pause Dogen said: This mind itself is Buddha is very difficult to understand. Mind is fences, walls, tiles, and pebbles, and Buddha is a glob of mud or a clump of soil. Kiangsi [Mazu] expressed trailing mud and dripping water; Damei realized lurking in the grasses and sticking to trees. Where can we find this mind itself is Buddha?
- from Dogen's Extensive Record, A Translation of the Eihei Koroku by Taigen Dan Leighton and Shohaku Okumura