Musnad Abu Ya'la al-Mawsili 118:40

Muhammad ibn Abd al-Rahman ibn Sahm al-Antaki narrated to us, Abdullah ibn al-Mubarak narrated to us, Ma'mar informed us, from al-Zuhri, from Anas, who said: The one among them who most resembled the Prophet, may Allah bless him and grant him peace, was al-Hasan ibn Ali.