Fornicators and adulterers are better off as Muslims

Sahih Al Bukhari: 6.139: Bk 65.137
Narrated `Abdullah: We used to participate in the holy wars carried on by the Prophet and we had no women (wives) with us. So we said (to the Prophet ). "Shall we castrate ourselves?" But the Prophet forbade us to do that and thenceforth he allowed us to marry a woman (temporarily) by giving her even a garment, and then he recited: "O you who believe! Do not make unlawful the good things which Allah has made lawful for you."

Sahih Muslim Bk 8, Number 3371: Bk 16.147
Abu Sirma said to Abu Sa'id al Khadri (Allah he pleased with him): 0 Abu Sa'id, did you hear Allah's Messenger (may peace be upon him) mentioning al−'azl? He said: Yes, and added: We went out with Allah's Messenger (may peace be upon him) on the expedition to the Bi'l−Mustaliq and took captive some excellent Arab women; and we desired them, for we were suffering from the absence of our wives, (but at the same time) we also desired ransom for them. So we decided to have sexual intercourse with them but by observing 'azl . But we said: We are doing an act whereas Allah's Messenger is amongst us; why not ask him? So we asked Allah's Mes− senger (may peace be upon him), and he said: It does not matter if you do not do it, for every soul that is to be born up to the Day of Resurrection will be born.

Chapter 29: IT IS PERMISSIBLE TO HAVE SEXUAL INTERCOURSE WITH A CAPTIVE WOMAN AFTER SHE IS PURIFIED (OF MENSES OR DELIVERY) IN CASE SHE HAS A HUSBAND, HER MARRIAGE IS ABROGATED AFTER SHE BECOMES CAPTIVE
Sahih Muslim Bk 8, Number 3432: Bk 17.41
Abu Sa'id al−Khudri (Allah her pleased with him) reported that at the Battle of Hanain Allah's Messenger (may peace be upon him) sent an army to Autas and encountered the enemy and fought with them. Having overcome them and taken them captives, the Companions of Allah's Messenger (may peace te upon him) seemed to refrain from having intercourse with captive women because of their husbands being polytheists. Then Allah, Most High, sent down regarding that:" And women already married, except those whom your right hands possess (iv. 24)" (i. e. they were lawful for them when their 'Idda period came to an end).

Sahih Muslim Bk 8, Number 3433: Bk 17.42-44
Abu Sa'id al−Khudri (Allah be pleased with him) reported that Allah's Apostle (may peace be upon him) sent a small army. The rest of the hadith is the same except this that he said: Except what your right hands possessout of them are lawful for you; and he did not mention" when their 'idda period comes to an end". This hadith has been reported on the authority of AbuSa'id (al−Khudri) (Allah be pleased with him) through another chain of transmitters and the words are: They took captives (women) on the day of Autas who had their husbands. They were afraid (to have sexual intercourse with them) when this verse was revealed:" And women already married except those whom you right hands posses" (iv. 24)

Sahih Muslim Bk 8, Number 3434: Bk 17.45
Qatada reported a hadith like this with the same chain of transmitters.

Surah 4:23,24
23. Prohibited to you are your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And the wives of your sons who are from your loins, and that you take two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.
24. And married women except those your right hands possess. the decree of Allah upon you. And lawful to you are beyond these, that you seek them with your property, desiring chastity, not unlawful sexual intercourse. So for whatever you enjoy from them, give them their due compensation as an obligation. And there is no blame upon you for what you mutually agree to beyond the obligation. Indeed, Allah is ever Knowing and Wise.

Sunan an-Nasai'i Vol 4, Hadith 3411: Bk 36.21
It was narrated from Anas, that the Messenger of Allah had a female slave with whom he had intercourse, but 'Aishah and Hafsah would not leave him alone until he said that she was forbidden for him. Then Allah, the Mighty and Sublime, revealed:
"O Prophet! Why do you forbid (for yourself) that which Allah has allowed to you.' until the end of the Verse.

Grade    : Sahih (Darussalam)    
Reference     : Sunan an-Nasa'i 3959
In-book reference     : Book 36, Hadith 21
English translation     : Vol. 4, Book 36, Hadith 3411