Hadith 8:3236
Abu Bakr narrated to us, he said: Mu'adh narrated to us, from Ash'ath, from al-Hasan, that he used to see no harm in the muhrim (a person in the state of ihram) wearing the qaba' (a type of outer garment/coat) provided he does not insert his shoulders into it.
حدثنا أبو بكر قال نا معاذ عن أشعث عن الحسن أنه كان لا يرى بأسا أن يلبس المحرم القبا لم يدخل منكبيه فيه