Farrah Abraham was apparently asleep at the wheel this weekend -- 'cause her car got jacked after she left the keys inside.

Law enforcement sources tell TMZ ... someone made off with a Hyundai Saturday that Farrah had rented. We're told she accidentally left her car keys in the vehicle, which she'd parked in a garage at a private residence in Bev Hills. Sure enough, it was boosted.

Unclear if the keys were in the ignition ... but we imagine it was a fairly easy getaway, regardless.

Luckily, this story's got a happy ending. Sources tell us cops spotted the car Monday night and ran the plates only to discover it was Farrah's stolen Hyundai. They pulled it over and arrested 3 men who were cruising in it. All three were booked for grand theft auto.