[Aloisius]

The Apple Watch Design Resources license where the font seems to have originated is pretty strict about using this font:

Limited License. Subject to the terms of this License, you may use the Apple Font solely for purposes of design and development of applications for the Apple Watch. The foregoing right includes the right to show the Apple Font in screen shots, images or mock-ups of an Apple Watch application.

https://developer.apple.com/watchkit/ (after clicking on Apple Watch Design Resources)