Musnad Ahmad 31:867

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Amr ibn Murrah, from Abu al-Bakhtari, from a man, from Abu Sa‘id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Let no one among you belittle himself. If he sees a matter for Allah in which he has a statement to make, yet he does not speak it, he will meet Allah having neglected that. So He will say: 'What prevented you?' He will say: 'I feared the people.' He will say: 'I was more deserving of being feared.'"