Musnad Ahmad 937:25

'Affan narrated to us, Hammam narrated to us, 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: "And what would you have asked him?" He said: "I would have asked him: Did he see his Lord?" He said: "I have indeed asked him, and he said: 'I saw a light; how could I see Him?'" 'Affan said: And it reached me from Ibn Hisham—meaning Mu'adh—that he narrated it from his father, just as Hammam said: "I saw Him."