Musnad Ahmad 31:792

Yazid ibn Harun narrated to us, Shu'bah informed us, from 'Amr ibn Murrah, from Abu al-Bakhtari, from a man, 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 prevent any one of you from speaking the truth when he witnesses it or knows it." Shu'bah said: I narrated this hadith to Qatadah, and he said: "What is this? 'Amr ibn Murrah, from Abu al-Bakhtari, from a man, from Abu Sa'id?" Abu Nadrah narrated to me, from Abu Sa'id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not the fear of people prevent any one of you from speaking the truth when he witnesses it or knows it." Abu Sa'id said: "This motivated me to ride to Mu'awiyah and fill his ears [with the truth], then I returned." Shu'bah said: Four people narrated this hadith to me from Abu Nadrah: Qatadah, Abu Maslamah, al-Jariri, and another man.