Musnad Ahmad 30:2464

Ibn Numayr narrated to us, he said: Ubaydullah narrated to us, from Sa’id al-Maqburi, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Were it not that I would make it difficult..." and he mentioned its meaning, and he said: "For Allah, the Almighty and Majestic, descends every night to the lowest heaven." And he said in it: "Until the dawn breaks."