1. You may not believe this, but the boss is quitting the company. The boss told me himself. I heard it straight from the horse’s mouth.
2. Tim asked Molly where she heard the news. He thought perhaps it was just a rumor, but Molly said she got it straight from the horse’s mouth.

Antonym: hear (something) through the grapevine. The expression is used to describe the source of information, usually of a rumor or of something that is not likely to be believed. The origin of the expression is the fact that one can always tell the true age of a horse from an examination of its teeth, i. e. from its mouth. Thus, the horse’s mouth is the most authoritative source of information about its age.