"Een gegeven paard moet men niet in de bek kijken"

Dutch saying: "Een gegeven paard moet men niet in de bek kijken"
Literal translation: "One should not look a given horse in the mouth".
Meaning: If you get something for free, you should not criticize it.
English equivalent: Beggars cannot be choosers" ("Do not look a given horse in the mouth" also exists, but I would prefer using a less literal translation).