Prophet Muhammad predicted that this question would one day be raised as he correctly predicted a great many future events of importance. On one occasion he said:
A day will certainly come when some people will sit with their legs crossed and ask: Given that God created everything, who created God? (Bukhari, Itisam, 3).
This question is derived from the observation of (what are taken to be) cause and effect relationships. Every circumstance can be thought of as an effect and attributed to an antecedent circumstance or cause which, in turn, is attributed to some circumstance antecedent to it, and so on. In the first place, it is obvious to anyone who reasons objectively that the notion of cause is only an hypothesis, it has no objective existence: all that objectively exists is a particular, often (but not always) repeated sequence of circumstances. Secondly, if this hypothesis is applied to existence as a whole, we cannot find a creator of it because each creator must have a creator before that creator, in a never-ending chain. (In fact, the futile notion of a never-ending chain of creators was one of the arguments used by Muslim theologians to explain the necessity of believing in God.)
So the Creator must be Self-Subsistent and One, without like or equal.