You use the phrase 'Don’t Look a Gift Horse in the Mouth' to say that it’s important to be grateful when a gift is received.
Example of use: “Be grateful you got a birthday present at all. Remember, don’t look a gift horse in the mouth.”
The expression 'don’t look a gift horse in the mouth' is an ancient one and its origin is unknown. It first appears in print in John Heywood’s A Dialogue Conteinyng The Nomber In Effect Of All The Prouerbes In The Englishe Tongue, where it translates to “Don’t look a given horse in the mouth.