Musnad Ahmad 31:823

Khalf ibn al-Walid narrated to us, 'Abbad ibn 'Abbad narrated to us, al-Mu'alla ibn Ziyad al-Qardusi narrated to us, 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 a man, if he knows a truth, from speaking it."