This short passage is taken from the Lankavatara sutra, an important source in Mahayana Buddhism.  It consists of a dialogue between the Buddha and a senior student named Mahamati. For other versions of a similar thought, see the portion of Shabistari’s poem posted here, and the Nasruddin version posted here.  

The reflection of form in a mirror
Is an image without substance.
Duality—like the image—
Is a projection of the mind.
The perception of external phenomena as reality
Is caused by diverse thoughts
Rooted in the psychic residue of past lives.
This is the transitory mind.
It creates all forms.
What appears to be external reality
Is actually nonexistent.
The seeming self within the body
Experiencing the senses
Is only the mind; this I proclaim.

Tr. Lobsang P. Lhalungpa