Arno Allan Penzias and Robert Woodrow Wilson were awarded (one half of) the 1978 Nobel prize in Physics, "for their discovery of cosmic microwave background radiation",

<https://www.nobelprize.org/prizes/physics/1978/summary/>

The story about their discovery is usually told with an anecdote about the pigeon droppings (allegedly described by Penzias as a "white dielectric") in their horn antenna, which was a cause of unwanted noise. I cannot find the origin of this story, presumably told by Penzias himself. Was this part of some interview or does it appear somewhere in a published (paper) form?