Sahih Muslim Book 1, Hadith Number 342.

Chapter : Pertaining to his (Prophet’s) words: He is a Light; how could I see Him?-and his words: I saw the Light.

Abdullah b. Shaqiq reported: I said to Abu Dharr: Had I seen the Messenger of Allah, I would have asked him. He (Abu Dharr) said: What is that thing that you wanted to inquire of him? He said: I wanted to ask him whether he had seen his Lord. Abu Dharr said: I, in fact, inquired of him, and he replied: I saw Light.