(straight) from the horse's mouth

Getting information from the highest authority.

Origin:
When you get a tip for a horse race, the tip is better as nearer it is to the horse e.g. the jockey or the trainer. When you got it straight from the horse's mouth, you have it directly from the source.

from the most official source(from the fact that a horse's age can be determined most precisely and directly by examining it's teeth)

from the highest authority