look a gift horse in the mouth - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.