ʿAbdullah b. Mas’ūd – Allah be pleased with him – said:
The believer sees his sins as if he is sitting at the foot of a mountain fearing that it might fall on him, while the sinner (fājir) sees his sins as a fly that lands on his nose, he just waves it away.
Al-Bukhārī, Al-Sahīh, The Book of Supplications, Chapter on Tawbah.
Ibn Hajr quotes in his commentary, Fath Al-Bārī:
Ibn Abī Jumrah said, “The reason for this [fear] is that the heart of a believer is illuminated; so when he sees from himself something that goes against what he illuminates his heart with, it is very distressing to him. The wisdom behind giving the example of a mountain is that a person might find some way to escape from other dangers, but if a mountain falls on a person he does not survive. In short, the believer is dominated by fear (of Allah) due to the strength of īmān he has; he does not therefore feel falsely secure about being punished because of his sins. This is the way of the Muslim: he always fears and checks on himself, his good deeds are little to him and he fears even the small bad deeds he has done.”