Euthyphro's Dilemma
Undermining the notion that objective morality is linked to Allah's commands, Euthyphro's dilemma is to be cited: "Is something morally good because Allah commands it, or does Allah command it because it's morally good?" By answering the question, theists either affirm that morality is defined by Allah's commands, or morality is external to His commands, and this is what Euthyphro's dilemma is about.
If theists say that "morality is based on Allah's commands," then what is good or evil is arbitrary, and in that case, there's nothing we as humans should necessarily recognise as objectively evil; this would imply that there is nothing intrinsically wrong with, say, killing children, just that Allah puts the "evil" label on it arbitrarily. Otherwise, the theists would resort to saying that "moral standards are completely outside and independent of Allah's essence and nature," however, relying on a standard external to Himself would imply that Allah is obligated to live by this standard, and He is not All-Powerful or independent, after all.
It may intuitively sounds like a valid contention, but a little reflection exposes it as a false dilemma due to a third possibility that wasn't mentioned at all; a morally stable and supreme being who would not arbitrarily change his mind about, say, the goodness of compassion and the evil of sexual misconduct. Allah is He, for He always commands good because His Character is naturally good.
What is being said here is that there's indeed a moral standard, but unlike what the second horn of the dilemma suggests, it's not external to Allah. Rather, it follows necessarily from Allah's nature. Muslims believe that Allah is necessarily and perfectly good; the maximal perfection of Allah means that His names and attributes are to the highest degree possible, with no deficiency or flaw.
As such, His nature contains within it the perfect non-arbitrary moral standard, and this means that an individual's actions, whatever they may be, are not arbitrarily bad, because they follow from an objective, necessary moral standard. On the other hand, it doesn't mean Allah is somehow subservient to this standard because it's contained in His essence, so it's not in any way external to Him.
One might object by saying: "You must know what good is to define Allah as good, therefore you haven't solved the problem." The reply to that is that while Allah's moral commands are a derivative of His will, His will does not contradict His nature of ultimate goodness. Allah is a Being of maximal perfection, a perfect being, thus perfect moral goodness is part of His essential nature. One of His beautiful names is Al-Barr (ٱلْبَرُّ) which means "The Source of Goodness." Part of Allah's maximal perfection is being morally perfect. His moral perfection is the standard for goodness. Therefore, what Allah commands is good because He is good, and He defines what good is:
And when they commit an immorality they say: "We found our fathers doing it and Allah hath enjoined it on us." Say: "Allah, verily, enjoineth not immorality. Tell ye concerning Allah that which ye know not?"
7:28
Allah enjoineth justice and kindness, and giving to kinsfolk, and forbiddeth lewdness and abomination and wickedness. He exhorteth you in order that ye may take heed.
16:90
Allah's commands are therefore the foundation of objective moral values, and His commands are based on His perfect nature. In summary, moral values are ultimately derivatives of Allah's will, expressed via His commands which don't contradict His perfectly good, wise and pure nature.