Jami` at-Tirmidhi Book 47, Hadith 386
the Messenger of Allah said: “Verily, when the slave (of Allah) commits a sin, a black spot appears on his heart. When he refrains from it, seeks forgiveness and repents, his heart is polished clean. But if he returns, it increases until it covers his entire heart. And that is the ‘Ran’ which Allah mentioned: ‘Nay, but on their hearts is the Ran which they used to earn.’”
The Messenger of Allah said: “Whoever wishes to look at the Day of Resurrection, as if he is seeing it with this eye, then let him recite: ‘When the sun Kuwwirat’ and ‘When the heaven is cleft sunder (Infatarat) and ‘When the heaven is split asunder.’”
The Day when mankind will stand before the Lord of all tat exists He said: “They will be standing in sweat up to the middle of their ear.” – Hammas said: To us it is Marfu - .