Muslems believe in the Qur'an (Koran). The Qur'an is considered by Muslems to be the literal and final word of their god "Allah".
Thus it makes sense to take a look at some relevant passages in the Qur'an to see what they choose to believe their god is telling them about how they should live and act.
002.223 - Your wives are as a tilth unto you; so approach your tilth when or how ye will [...]
003.028 - Let not the believers take for friends or helpers Unbelievers rather than believers [...]
003.054 - And they (the disbelievers) schemed, and Allah schemed (against them): and Allah is the best of schemers.
004.025 - If any of you have not the means wherewith to wed free believing women, they may wed believing girls from among those whom your right hands possess: And Allah hath full knowledge about your faith. Ye are one from another: Wed them with the leave of their owners, and give them their dowers, according to what is reasonable: They should be chaste, not lustful, nor taking paramours: when they are taken in wedlock, if they fall into shame, their punishment is half that for free women. This (permission) is for those among you who fear sin; but it is better for you that ye practise self-restraint.
004.034 - [on women] Men are the maintainers of women because Allah has made some of them to excel others and because they spend out of their property; the good women are therefore obedient, guarding the unseen as Allah has guarded; and (as to) those on whose part you fear desertion, admonish them, and leave them alone in the sleeping-places and beat them; then if they obey you, do not seek a way against them; surely Allah is High, Great.
004.089 - [on unbelievers] They but wish that ye should reject Faith, as they do, and thus be on the same footing (as they): But take not friends from their ranks until they flee in the way of Allah (From what is forbidden). But if they turn renegades, seize them and slay them wherever ye find them; and (in any case) take no friends or helpers from their ranks.
004.091 - Others you will find that wish to gain your confidence as well as that of their people: Every time they are sent back to temptation, they succumb thereto: if they withdraw not from you nor give you (guarantees) of peace besides restraining their hands, seize them and slay them wherever ye get them: In their case We have provided you with a clear argument against them.
004.144 - Take not for friends unbelievers rather than believers: Do ye wish to offer Allah an open proof against yourselves?
005.044 - [...] And if any fail to judge by (the light of) what Allah hath revealed [i.e., follow the Koran], they are (No better than) wrong-doers.
009.111 - Allah hath bought from the believers their lives and their wealth because the Garden will be theirs: they shall fight in the way of Allah and shall slay and be slain.
016.106 - Any one who, after accepting faith in Allah, utters Unbelief,- except under compulsion, his heart remaining firm in Faith - but such as open their breast to Unbelief, on them is Wrath from Allah, and theirs will be a dreadful Penalty.
025.052 - So obey not the disbelievers, but strive against them herewith with a great endeavour.
033.059 - Tell thy wives and daughters, and the believing women, that they should cast their outer garments [headscarfs] over their persons (when abroad): that is most convenient, that they should be known (as such) and not molested.
043.025 - So We exacted retribution from them: now see what was the end of those who rejected (Truth)!
047.004 - Therefore, when ye meet the Unbelievers (in fight), smite at their necks; At length, when ye have thoroughly subdued them, bind a bond firmly (on them): thereafter (is the time for) either generosity or ransom: Until the war lays down its burdens. Thus (are ye commanded): but if it had been Allah's Will, He could certainly have exacted retribution from them (Himself); but (He lets you fight) in order to test you, some with others. But those who are slain in the Way of Allah,- He will never let their deeds be lost.
047.035 - Be not weary and faint-hearted, crying for peace, when ye should be uppermost [...]
061.009 - It is He Who has sent His Messenger with Guidance and the Religion of Truth, that he may proclaim it over all religion[s], even though the Pagans may detest (it).