Musnad Ahmad 31:830

Khalaf ibn al-Walid narrated to us, Khalid narrated to us, from al-Jurayri, from Abu Nadra, 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."