Straight from the horse's mouth Informal

Information obtained directly from the original, most authoritative, or most reliable source, rather than through rumor or secondhand accounts.

It’s usually said to reassure someone that information can be trusted, precisely because of where it came from.

The phrase comes from horse racing and horse trading, where a horse’s true age can be determined with real accuracy by examining the growth and wear of its teeth, a much more reliable method than trusting a seller’s word, which sellers had every incentive to exaggerate favorably. Getting information “straight from the horse’s mouth,” then, originally meant literally checking the horse’s teeth yourself rather than taking anyone’s claims about the animal on faith. The phrase is documented in this figurative sense, for any firsthand, unimpeachable source of information, from the early twentieth century, closely tied to the culture of horse racing and betting where trustworthy information was, quite literally, worth money.

I got this straight from the horse’s mouth, so it’s definitely true.

She heard it straight from the horse’s mouth: the merger is happening next month.

Don’t trust rumors, get it straight from the horse’s mouth.

A: Is he really leaving the company? B: Yes, I heard it straight from the horse’s mouth.