Musnad Ahmad 33:176

'Affan narrated to us, Aban al-'Attar narrated to us, Yahya ibn Abi Kathir narrated to us, he said: I asked Abu Salamah ibn 'Abd al-Rahman, "Which of the Qur'an was revealed first?" He said: "O you who are wrapped in garments" (Al-Muddaththir). He mentioned the hadith, except that he said: "When I finished my retreat, I descended and went into the valley, and I was called." He also mentioned, he said: "I looked above me, and there he was, sitting on a throne between the heaven and the earth. I was struck with fear of him, so I came to Khadijah's house and said: 'Wrap me up, wrap me up.'" He then mentioned the rest of the hadith.