Imam Sharaf ad-Deen at –Taybee (d. 743 A.H.) -Rahimullaah- said in his explanation of Mishkat al-Masabeeh (p.739):
‘Regarding his –sallAllaahu alayhi wa sallam– saying: ‘and the prayer is Noor (light)’ it means that the prayer prevents a person from sinning and prohibits evil sexual and horrible deeds, and it guides to what is correct just like light.
It has been said, that what it means by light, it is the thing with which the one who prays will be guided by on the Day of Judgment.
Allaah The Most High said:
يَوْمَ تَرَى الْمُؤْمِنِينَ وَالْمُؤْمِنَاتِ يَسْعَىٰ نُورُهُم بَيْنَ أَيْدِيهِمْ وَبِأَيْمَانِهِم بُشْرَاكُمُ الْيَوْمَ جَنَّاتٌ تَجْرِي مِن تَحْتِهَا الْأَنْهَارُ خَالِدِينَ فِيهَا ۚ ذَٰلِكَ هُوَ الْفَوْزُ الْعَظِيمُ
<<On the Day you shall see the believing men and the believing women their light running forward before them and by their right hands. Glad tidings for you this Day! Gardens under which rivers flow (Paradise), to dwell therein forever! Truly, this is the great success! >> [Hadeed: 12]
It has been said that the prayer is light because the prayer is the means that enlighten the lights (Noor) of good deeds, and the delight of the heart, and uncovers realities due to the heart being concerned with the prayer, and turning to Allaah inwardly and outwardly. And it has been said that: Noor is the sign on the face of the one who prays, due to the effects of prostration.’
[‘Tayseer Rab al-Bariya fee Sharh al-Arba’een an-Nawawi’ p.341]
Shaykh Muhammed bin Salih al-Uthaymeen (d.1421 A.H.) said:
‘Regarding the saying of Messenger –sallAllaahu alayhi wa sallam– ‘and the prayer is light (Noor)’ i.e. that the obligatory and optional prayer is light, Noor in the heart, Noor on the face, Noor in the grave, Noor on the day of Hashr, because the hadeeth is absolute, and the one who experiences this will find that Noor.
If you prayed the prayer in reality, in that which you bring your heart to attention to, and your limbs to have Khushoo, you will feel that your heart has become enlightened and it has tasted delight, absolute delight, this is why the Prophet –sallAllaahu alayhi wa sallam– said: ‘The sweetness/pleasure of my eyes has been placed in the prayer.’ [Ahmad]
[Taken from ‘Sharh al-Arbaeen an-Nawawi’ – Shaykh Muhammad bin Salih al-Uthaymeen p.246 Hadeeth No.23]
Source: Benefits & Principles From The Forty Ahadeeth Of Imam Nawawi – Hadeeth No.23. Compiled by Abbas Abu Yahya (hafidhahullaah)