Musnad Ahmad 937:237

Yazid—meaning Ibn Harun—narrated to us, Yazid ibn Ibrahim narrated to us, Qatadah narrated to us, Abdullah ibn Shaqiq narrated to us, he said: I said to Abu Dharr: "If I had reached the Prophet, may Allah bless him and grant him peace, I would have asked him." He said: "And what would you have asked him about?" He said: "I would have asked him: Did he see his Lord, the Almighty and Majestic?" Abu Dharr said: "I have already asked him, and he said: 'Light, how could I see Him?'"