One should recite the following dua when looking at one’s face in the mirror:
اَلْحَمْدُ لِلّٰهِ الَّذِي سَوَّى خَلْقِي فَعَدَلَهُ، وَصَوَّرَ صُورَةَ وَجْهِي فَحَسَّنَهَا، وَجَعَلَنِي مِنَ الْمُسْلِمِينَ
All praise is due to Allah Ta‘ala, Who created my composition in due proportion and He created it with perfect proportion and balance, and He gave my face its form and beautified it (for me), and He made me from among the Muslims
عن أنس بن مالك قال: كان رسول الله صلى الله عليه وسلم إذا نظر وجهه في المرآة قال: الحمد لله الذي سوى خلقي فعدله، وصور صورة وجهي فحسنها، وجعلني من المسلمين (المعجم الأوسط، الرقم: 787)
Hazrat Anas (radhiyallahu ‘anhu) reports that Rasulullah (sallallahu ‘alaihi wasallam) would recite the following dua when he would look at his mubaarak face in the mirror:
اَلْحَمْدُ لِلّٰهِ الَّذِي سَوَّى خَلْقِي فَعَدَلَهُ، وَصَوَّرَ صُورَةَ وَجْهِي فَحَسَّنَهَا، وَجَعَلَنِي مِنَ الْمُسْلِمِينَ