Unresolved external symbol on static class members

// test.h
class CTest
{
public:
    int getNumber()
    {
        return m_snInt;
    }
    void setNumber(int n)
    {
        m_snInt = n;
    }
private:
    int m_nInt;
    static int m_snInt;
};

// test.cpp
int CTest::m_snInt;
int CTest::getNumber()
{
    // TO-DO
}