The Messenger of Allah (ﷺ) used to say:

اَللَّهُمَّ أَحْسَنْتَ خَلْقِيْ فَأَحْسِنْ خُلُقِيْ

Allaahumma a’hsanta khalqi fa a’hsin khuluqi

O Allah, you have perfected my creation, so too perfect my character.

Reference: Sahih (Albani). Ahmad: 24392