Musnad Ahmad 31:687

'Affan narrated to us, Hammad narrated to us, from 'Ali ibn Zayd, from al-Hasan, that Abu Sa'id al-Khudri said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not the awe of people prevent a man from standing up for the truth when he knows it." Abu Sa'id then wept and said: "By Allah, we have witnessed it, yet we did not stand up for it."