Sahih Bukhari Volume 008, Book 075, Hadith Number 320.
Narated By Al-Harith bin Suwaid : 'Abdullah bin Mas'ud related to us two narrations: One from the Prophet and the other from himself, saying: A believer sees his sins as if he were sitting under a mountain which, he is afraid, may fall on him; whereas the wicked person considers his sins as flies passing over his nose and he just drives them away like this." Abu Shihab (the sub-narrator) moved his hand over his nose in illustration. (Ibn Mas'ud added): Allah's Apostle said, "Allah is more pleased with the repentance of His slave than a man who encamps at a place where his life is jeopardized, but he has his riding beast carrying his food and water. He then rests his head and sleeps for a short while and wakes to find his riding beast gone. (He starts looking for it) and suffers from severe heat and thirst or what Allah wished (him to suffer from). He then says, 'I will go back to my place.' He returns and sleeps again, and then (getting up), he raises his head to find his riding beast standing beside him."