Formless

"The Chán tradition ascribes the origins of Chán in India to the Flower Sermon, the earliest source for which comes from the 14th century. It is said that Gautama Buddha gathered his disciples one day for a Dharma talk. When they gathered together, The Buddha was completely silent and some speculated that perhaps The Buddha was tired or ill. The Buddha silently held up and twirled a flower and his eyes twinkled; several of his disciples tried to interpret what this meant, though none of them were correct. One of The Buddha's disciples, Mahākāśyapa, silently gazed at the flower and broke into a broad smile. The Buddha then acknowledged Mahākāśyapa's insight by saying the following:

I possess the true Dharma eye,
the marvelous mind of Nirvāṇa,
the true form of the formless,
the subtle Dharma gate that does not
rest on words or letters but is a special
transmission outside of the scriptures.
This I entrust to Mahākāśyapa."

- The Flower Sermon
Quoted from the Chinese Buddhist Encyclopedia