moral fibre
noun /ˌmɒrəl ˈfaɪbə(r)/
  /ˌmɔːrəl ˈfaɪbər/
 (US English moral fiber)
 [uncountable]- the inner strength to do what you believe to be right in difficult situations- It isn’t just a lack of moral fibre that leads to a rising divorce rate.