Don't look a gift horse in the mouth

Don't be ungrateful or overly critical when receiving something for free; accept generosity graciously without examining it too closely for flaws.

This proverb dates back to the 4th century CE and comes from the practice of determining a horse's age and health by examining its teeth. Since a horse's teeth wear down and change with age, looking in a gift horse's mouth would be checking its value - essentially questioning the giver's generosity.