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Phrase(s): look a gift horse in the mouth
Fig. to be ungrateful to someone who gives you something; to treat someone who gives you a gift badly. (Usually with a negative.) • Never look a gift horse in the mouth. • I advise you not to look a gift horse in the mouth.
McGrawhill's American Idioms And Phrasal Verbs
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Be critical or suspicious of something received at no cost. For example, Dad's old car is full of dents, but we shouldn't look a gift horse in the mouth. This term, generally expressed as a cautionary proverb (Don't look a gift horse in the mouth), has been traced to the writings of the 4th-century cleric, St. Jerome, and has appeared in English since about 1500. It alludes to determining the age of a horse by looking at its teeth.
American Heritage Idioms