Accayaṃ desayantīnaṃ yo ca na paṭiggaṇhati
Kopan-taro dosagaru, so veraṃ paṭimaññati.

When someone apologizes,
if a person remains resentful, filled with anger, and refuses to accept it,
that person is said to be harboring enmity.
(Saṃyutta Nikāya, 15/110)