Allah Subhanawataala says in the Hadith Qudsi, what is the sin of omission……..
On the authority of Abu Sa’id (may Allah be pleased with him), who said that the Messenger of Allah (PBUH) 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 say: [It was] out of fear of people. Then He says: Rather it is I whom you should more properly fear.
It was related by Ibn Majah with a sound chain of authorities.