Musnad Ahmad 31:798

Ibn Abi ‘Adi narrated to us, from Humayd, from Bakr al-Muzani, who said: Abu Sa’id al-Khudri said: I saw a dream while I was writing Surah Sad. He said: When I reached the prostration, I saw the inkwell, the pen, and everything in my presence turn prostrating. He said: So I narrated it to the Messenger of Allah, peace and blessings of Allah be upon him, and he did not cease to prostrate during it.