don't look a gift horse in the mouth
Since horses' teeth grow over time, checking their length is a way of gauging old age, and therefore a sign of mistrust towards the giver.
- He ne’er consider'd it, as loth
- To look a Gift-horse in the mouth.
a phrase referring to unappreciatively questioning of a gift or handout too closely
- Gregory Y. Titelman, Random House Dictionary of Popular Proverbs and Sayings, 1996, ISBN 0-679-44554-4, p. 69.