Bukhari, Volume 8, Book 82, Number 812:
(this is in the section dealing with adultery, so while the crime remains unexpressed, it has been understood to be illegal sexual intercourse -- adultery or pre-marital relations)

Narrated Anas bin Malik:

While I was with the Prophet a man came and said, "O Allah's Apostle! I have committed a legally punishable sin; please inflict the legal punishment on me'." The Prophet did not ask him what he had done. Then the time for the prayer became due and the man offered prayer along with the Prophet , and when the Prophet had finished his prayer, the man again got up and said, "O Allah's Apostle! I have committed a legally punishable sin; please inflict the punishment on me according to Allah's Laws."

The Prophet said, "Haven't you prayed with us?"

He said, "Yes."

The Prophet said, "Allah has forgiven your sin."

and:

Bukhari,Volume 7, Book 72, Number 717:
Narrated Abu Dharr:

I came to the Prophet while he was wearing white clothes and sleeping. Then I went back to him again after he had got up from his sleep.

He said, "Nobody says: 'None has the right to be worshipped but Allah' and then later on dies while believing in that, except that he will enter Paradise."

I said, "Even if he had committed illegal sexual intercourse and theft?"

He said. "Even if he had committed illegal sexual intercourse and theft."

I said again, "Even if he had commited illegal sexual intercourse and theft?"

He said. "Even if he had committed illegal sexual intercourse and theft."

I said again, "Even it he had committed illegal sexual intercourse and theft!"

He said, "Even If he had committed Illegal sexual intercourse and theft, and despite Abu Dharr's not liking it."

Abu 'Abdullah said, "This is at the time of death or before it if one repents and regrets and says "None has the right to be worshipped but Allah. He will be forgiven his sins."