One day, at the Mount of the Holy Vulture, Sakyamuni Buddha hold up a flower to the assembly and said nothing. Nobody understood the meaning of this. Only Mahakasyapa smiled and did not say anything either.
Thereupon, the Buddha solemnly proclaimed: "I have the treasure of the correct Dharma eye, Nirvana’s wonderful mind which I now hand over to Mahakasyapa."
A special transmission outside the scriptures,
Not founded upon words and letters;
By pointing directly to [one’s] mind,
It lets one see into [one’s own] nature and [thus] attain Buddhahood.
Bodhidharma, the Twenty Eighth Patriarch in the Indian Tradition and the First Patriarch in the Chinese Tradition
“What is the meaning of tso ch’an (sitting in meditation)?” the Master asked the assembly. “In this teaching, it means a mind without barriers and obstructions. ‘Sitting’ (tso) means outwardly and under all circumstances —be they good or otherwise, not to have any thought arise. ‘Meditation’ (ch’an) is internally to realize the imperturbability of self-nature.”
The Sixth Patriarch Hui Neng, The Platform Sutra