Volume 3, Book 43, Number 655:
Narrated Abu Huraira (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) said, “When an adulterer commits illegal sexual intercourse, then he is not a believer at the time he is doing it, and when a drinker of an alcoholic liquor drinks it, then he is not a believer at the time of drinking it, and when a thief steals, then he is not a believer at the time of stealing, and when a robber robs, and the people look at him, then he is not a believer at the time of doing robbery.
A believer who indulges in a major sin cannot possibly have the fear/consciousness of Allah while he’s committing the sin. Iman increases and decreases, and sometimes it just goes too low.
Only sincere repentance can wipe out these sins from our slate, nothing else.
Al-Tirmidhi narrated that Anas ibn Malik (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: “Allah, may He be blessed and exalted, said: ‘O son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind. O son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I would forgive you and I would not mind.’” [Saheeh al-Tirmidhi]