Musnad Ahmad 31:484

Muhammad ibn al-Hasan narrated to us, Ja'far narrated to us, from al-Mu'alla al-Qardusi, from al-Hasan, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Beware, let not the fear of people prevent any one of you from speaking the truth when he sees it or witnesses it. For it does not bring near an appointed time (death), nor does it distance a provision, that one should speak the truth or mention a great matter."