Musnad Abu Ya'la al-Mawsili 42:239

Abu Khaythama narrated to us, Abd al-Samad ibn Abd al-Warith narrated to us, al-Mustamir ibn al-Rayyan al-Ayadi narrated to us, Abu Nadra narrated to us, from Abu Sa’id, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Let not the fear of people prevent any one of you from speaking the truth when he sees it and knows it, or when he sees it and hears it."