Ayat 15 of Surat Ma'ida of the Holy Qur'an (5:15) reads:--
"From Allah has come to you a Light and a Book manifest."
Qadi 'Iyad, may Allah be Merciful to him, said: "He (Sallallahu alaihi Wasallam) was named a light because of the clarity of his case and the fact that his Prophecy was made manifest, and also because of the illumination of the hearts of the believers and the knowers of Allah with what he brought."
Mulla Ali Qari said in Sharh al-Shifa (1:505, Mecca ed.): "It has also been said that both the light and the book refer to Muhammad, because just as he is a tremendous light and the source of all lights, he is also a book that gathers up and makes clear all the secrets." He also said (1:114, Madina ed.): "And what objection is there to predicate both nouns to the Prophet, since he is in truth an immense Light due to the perfection of his appearance among all light, and he is a manifest Book since he gathers up the totality of secrets and he makes evident all Laws, situations and alternatives."