Musnad Ahmad 31:450

Waki’ narrated to us, from Sufyan, and ‘Abd al-Razzaq said: 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, by saying [something] about it, so it is said to him on the Day of Resurrection: 'What prevented you from speaking about it?' He will say: 'O Lord, I feared the people.' He will say: 'But I am more worthy that you should fear Me.'" And Abu Nu’aym said—meaning in the hadith—: "And I was more worthy that you should fear Me."