Musnad Ahmad 31:748

'Affan narrated to us, Yazid—meaning Ibn Zuray'—narrated to us, Humayd narrated to us, he said: Bakr narrated to me, that he was informed that Abu Sa'id al-Khudri saw a dream that he was writing [Surah] Sad. When he reached its prostration, he said: He saw the inkwell, the pen, and everything in his presence, turn prostrating. He said: So he narrated it to the Prophet, may Allah bless him and grant him peace, and he did not cease to prostrate during it thereafter.