The U.N. Independent International Commission of Inquiry on Syria published its findings on the atrocities committed by Islamic State. They have published a paper called "Rule of Terror: Living Under ISIS in Syria" which documents the ISIS tactics of killing, rape, enslavement, and sexual slavery. It defines these as war crimes and crimes against humanity.
Anyone with a reasonable knowledge of Muhammad's life will know the following account:
In March 627 AD, the tribe known as the Banu Qurayzah were besieged and isolated by their Muslim attackers led by Muhammad. They twice offered to leave their stronghold but Muhammad refused their request. He insisted they surrender unconditionally and subject themselves to his judgement. Compelled to surrender, the Qurayzah were led to Medina. A third (and final) appeal for leniency was made to Muhammad by their tribal allies, the Aus. Again Muhammad refused. Instead he orchestrated a sentence of death to be placed on the Banu Qurayzah by appointing a man with a grudge against them to pass judgement.
About 800 men were led to trenches dug in the Market of Medina and there they were beheaded, their decapitated bodies buried in the trenches while Muhammad watched. Male youths who had not reached puberty were spared. Women and children were sold into slavery, some being distributed as gifts among Muhammad’s companions. Muhammad himself took the most beautiful as his sex slave.
A Chechen jihadist, Abu Muhammad Ar-Rusi, has recently justified the behaviour of IS with reference to this exact same event.
The logic is therefore perfectly clear:
- The UN body defines these acts [mass executions, sexual slavery, rape, and enslavement] as war crimes and crimes against humanity;
- Muhammad committed exactly these acts;
- Therefore Muhammad committed war crimes and crimes against humanity.
 The Life of Muhammad: A Translation of Ibn Ishaq's Sirat Rasul Allah, A. Guillaume, translator, (Oxford University Press, 1955) pp. 468-469
 For an overview of the literature: http://www.hirhome.com/islam/art.htm
 The Quran refers obliquely to the massacre: "And those of the People of the Book who aided them — Allah did take them down from their strongholds and cast terror into their hearts. (So that) some ye slew, and some ye made prisoners" (Quran 33:26). And Muhammad again delivered revelations ascribing victory to Allah alone (Quran 33:9-11)
See Robert Spencer: The Truth about Muhammad pp.130-131