darovanému koni na zuby nekoukej

Etymology
lit. 'don't look at the teeth of a gift horse'. 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. Although uncertain, the origin can be traced even further to St. Jerome's Latin translation equi donati dentes non inspiciuntur, from the Letter to the Ephesians, circa AD 400.

Proverb

 * 1) don't look a gift horse in the mouth