Agent Six has provided a verifiable example from 1976.
The sarcastic interjection in your (or my, her, his, etc.) dreams is familiar in everyday spoken English, but the earliest evidence our editors have found comes from a Usenet post in 1986.
We suspect that it may have been used earlier than that; can you help us prove it? Popular culture such as film and television might be a good place to start.
Here is our entry for reference:
OED editor Fiona McPherson explains this appeal below:
Posted by OED_Editor on 26 September 2012 20.01