Musnad Ahmad 31:438

'Abd al-Samad narrated to us, al-Mustamir narrated to us, Abu Nadra narrated to us, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not the fear of people or a human being prevent any one of you from speaking the truth when he sees it, or knows it, or sees it, or hears it."