Musnad Ahmad 31:708

'Abd al-Razzaq narrated to us, Sufyan informed us, from Zubayd, from 'Amr ibn Murrah, from Abu al-Bakhtari, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let no one among you belittle himself when he sees a matter for Allah in which he should speak, yet he does not speak about it. Then it will be said to him on the Day of Resurrection: 'What prevented you from speaking about such and such?' He will say: 'Fear of the people.' He will say: 'It is Me that you should have feared more.'"