Musnad Ahmad 27:1806

Muhammad ibn Abd al-Rahman al-Tafawi narrated to us, Ayyub narrated to us, from Nafi’, that Ibn Umar went out for Hajj and entered into Ihram. He placed his head in severe cold, so I threw a burnous (hooded cloak) over him. He woke up and said, "What did you throw over me?" I said, "A burnous." He said, "You throw it over me when I have narrated to you that the Messenger of Allah (peace and blessings of Allah be upon him) forbade us from wearing it?"