‘Abdullah b. Mas’ûd – 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 (fâjir) sees his sins as a fly that lands on his nose, he just waves it away.
Al-Bukhârî, Al-Sahîh, The Book of Supplications, Chapter on Tawbah.

Ibn Hajr quotes in his commentary, Fath Al-Bârî:Ibn Abî 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 îmân 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.”
It’s just a small sin

Bilâl b. Sa’d – Allah have mercy on him – said:

Do not think about how small the sin is, but think about who you have just disobeyed.
Ibn Al-Mubârak, Al-Zuhd wa Al-Raqâ`iq Vol.1 p150.

Source: http://www.sayingsofthesalaf.net/
https://abdurrahman.org/2009/09/29/a-fly-or-a-mountain/