Musnad Ahmad 937:209

Yahya ibn Sa'id narrated to us, from Yazid ibn Ibrahim, Qatadah narrated to us, from 'Abdullah ibn Shaqiq, who said: I said to Abu Dharr: "If I had seen the Messenger of Allah, may Allah bless him and grant him peace, I would have asked him." He said: "About what thing?" I said: "I would ask him: Did Muhammad see his Lord?" He said: "He replied: I have already asked him, and he said: 'Light, how could I see Him?'"