(Warning: biased source)
Electrek has an article quoting it. Elon was describing the idea of designing the "Machine that builds the Machines"[0].
“We realized that the true problem, the true difficulty, and where the greatest potential is – is building the machine that makes the machine. In other words, it’s building the factory. I’m really thinking of the factory like a product.”
[0]:https://electrek.co/2016/06/01/elon-musk-machines-making-mac...
Can't find the article but I was an interesting read