Now what, Ānanda, is the recognition of selflessness? Here, Ānanda, a monk, gone to the wilderness, to the root of a tree, or to an empty place, discriminates thus: ‘The eye is not-self, forms are not-self; the ear is not-self, sounds are not-self; the nose is not-self, odors are not-self; the tongue is not-self, flavors are not-self; the body is not-self, tactual objects are not-self; the mind is not-self, phenomena are not-self.’ Thus he abides contemplating selflessness with regard to the six internal and external sensory spheres. This, Ānanda, is called the recognition of selflessness.
Buddha, Girimānanda Sutta
Buddha, Girimānanda Sutta