On the authority of Abu Sa'id (may Allah be pleased with him), who said that the Messenger of Allah (peace and blessings of Allah be upon him) said:

"Let not any one of you belittle himself." They said: "O Messenger of Allah, how can any one of us belittle himself?" He said: "He finds a matter concerning Allah about which he should say something, and he does not say [it], so Allah (mighty and sublime be He) says to him on the Day of Resurrection: 'What prevented you from saying something about such-and-such and such-and-such?' He will say: 'It was] out of fear of people.' Then He says: 'Rather it is I whom you should more properly fear.'"

[Ibn Majah with a sound chain of authorities.]