Static const getter file giving me an error

Question

Item.h:

class Item
{
private:
    static const int price = 5;
public:
    static const int& getPrice();
};

Item.cpp:

const int& Item::getPrice(){
    return price;
}

main.cpp:

int main()
{
    Item item;
    cout << item.getPrice() << endl;
}

Error: undefined reference to 'Item::price'

Why is this?