On Dreams (Greek: Περὶ ἐνυπνίων, Latin: De insomniis) is a work by Aristotle, one of the short treatises on nature, or Parva Naturalia.
Free online texts
Internet Archive: The Works of Aristotle, Vol III, translated under the editorship of W.D. Ross (1910). Parva Naturalia translated by J.I. Beare and G.R.T. Ross. Multiple formats.
Internet Archive: Aristotle's Psychology: A Treatise on the Principle of Life (De Anima and Parva Naturalia), translated by W.A. Hammond (1902). Multiple formats.
Mikrosapoplous.gr: ΠΕΡΙ ΕΝΥΠΝΙΩΝ. Greek text, HTML format.
University of Adelaide (Internet Archive): On Dreams, translated by J.I. Beare. Multiple formats.
Librivox: Parva Naturalia - public domain audiobook.
Wikipedia: On Dreams
The Great Conversation: Further reading at Tom's Learning Notes
Ancient Greek resources: Learn to read Greek classics in the original.