Musnad Ahmad 31:413

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu Maslamah, that he heard Abu Nadrah narrating from Abu Sa'id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, that he said: "Let not the fear of people prevent any one of you from speaking the truth when he sees it or knows it."