The connection between light on the face and Salaah

 “His statement (ﷺ): “And Salaah is light”

“So the prayer is light: light for the servant in his heart, on his face, in his grave, on the day of judgement when he is brought back and due to this you find most of the people with light on their faces is those who constantly perform Salaah and have the most Khushoo(Humility) in it for Allah mighty and majestic.”

From The explanation of Riyaadh Saliheen of Shaykh Ibn Uthaymeen May Allah have mercy upon him page.190 Volume One

Translator Abu Dhahhak Abdurrahman Alex: [The original hadeeth is found in Saheeh Muslim]

“قوله: “والصلاة نُور

فالصلاة نورٌ: نوٌر للعبد في قلبه، و في وجْهه، و في قبْرهِ، و في حَشْرِه، و لهاذا تجد اكثر الناس نورًا في الوجوهِ أكثرَهُم صلاةً، و أخشعهم فيها لله عزَّ و جلَّ.

من شرح رياض الصالحين للشيخ إبن عثيمين رحمه الله   ص ١٩٠ المجلد الأول