Musnad Ahmad 16:2

Waki’ narrated to us, from Sharik, from Abu Ishaq, from Hubayrah, who said: Al-Hasan ibn Ali, may Allah be pleased with him, delivered a sermon to us and said: "A man has departed from you yesterday, whom the first ones did not surpass in knowledge, nor will the last ones attain him. The Messenger of Allah, may Allah bless him and grant him peace, used to send him with the banner, with Jibril on his right and Mika'il on his left; he would not turn back until it was opened for him."