Here is the summary of Avicenna’s (Ibn Sina) argument for existence of God:

For all that exists or does not exist fall into these categories:
Impossible being – (ghair mumkin or mumtani’)
Possibly Existent or Contingent (imkan al-wujud)
Necessary Existent (wajib al-wujud)

• A contingent being exists.
• This contingent beings has a cause or explanation of the its
  existence.
• The cause or explanation of its existence is something other
  than the contingent being itself (if it were, it also would need
  an explanation leading to infinite regress)
• What causes or explains the existence of this contingent
  being must either be other contingent beings or a noncontingent
  being (that is, a necessary being)
• Contingent beings alone cannot ultimately cause or explain
  the existence of a contingent being.
• Therefore, what causes and explains the existence of this
  contingent being must include a noncontingent, necessary
  being
• Therefore, a necessary being (God) exists.

Avicenna says: “that which is impossible for it to be will not be, and that which is contingent (mumkin) for it to be, has its contingency [of being] prior to its existence/being” (al-muhal an yujad las yujad, wal-mumkin an yujad qad sabaqahu imkan wujudih).  Avicenna, Kitab al-Najat, Metaphysics I, ed. by Jajed Fakhry p. 255

“The full argument is a bit complicated, but here is a somewhat simplified version. Avicenna’s proof actually has nothing to do with design, he doesn’t need the idea that the universe is intelligently put together. Instead, he argues from the idea that the things we see around us are ‘contingent’ or merely ‘possible’.

“The idea here is that a contingent thing is something that may either exist or not exist; its nature does not guarantee that it exists. What Avicenna wants to do is show you that although all the things we experience directly are indeed contingent, there is also something else that exists necessarily, in other words, whose very nature guarantees that it exists.

“To do this, Avicenna points out that since a contingent thing on its own merit could either exist or not exist, it must have some external cause that made it exist – like ‘tipping the scales’ in favor of its existence rather than its non-existence.

“So take me, for instance. I am contingent, meaning that I am the sort of thing that could easily have failed to exist. In fact, at one time I didn’t yet exist, and in the future I will cease existing, that proves I’m not necessary.

“So there must have been a cause, maybe my parents, who brought me into existence. Now Avicenna observes that the aggregate whole of all contingent things – in other words the physical universe – is also contingent. After all, everything in the universe is contingent, so taken all together as one thing, it too must be contingent. Thus it also needs an external cause, just like I do.

“Since that external cause has to be outside the whole aggregate of contingent things, it cannot itself be contingent. So it is necessary. Hey presto, we’ve proven that there is a necessary existent which causes all other things! And this, of course, is God.”

“One thing I like about this proof is that it captures, in rigorous terms, a reason that I think actually underlies people’s belief in God. Effectively, Avicenna is trying to show that when you look around and think, ‘All of this could have failed to exist; why is there something, rather than nothing?’ you are asking a good question.

“The answer to the question is that not everything can be contingent; that is, not everything could have failed to exist. There must be something that just has to exist, to explain why everything else has wound up existing.” (Philosophy in the Islamic World – Peter Adamson)