Where (preferably online) is the passage of Rabbeinu Bahya Ibn Paqud in his Chovos Halevavos (in Shaar HaYichud I believe) in which he proves the existence of the Creator by citing a story where a comparison is made to the ability to detect non-randomness in a well written poem (i.e. that it was not created accidentally by spilt ink)?
shaar yichud ch.7 online
The analogy of this: When one sees a letter of uniform handwriting and writing style, one will immediately consider that one person wrote it because it is not possible that there was not at least one person. If it were possible that it could have been written with less than one person, we would consider this possibility. And even though it is possible that it was written by more than one person, it is not proper to consider this, unless there is evidence which testifies to this, such as different handwriting style in part of the letter or the like.
Since this is so, it is not necessary to know Him face to face, if this is not possible, and it will suffice for us to see the letter, accepting as proof the writer's acts, namely, the form of the writing, instead of seeing the writer himself. From this alone, we will know clearly that one writer exists, who knows how to write and is capable of writing, and that he wrote this letter, and did not partner with someone else in writing it. This we can see from its orderly form and uniform handwriting, since the handiwork of two makers is changing, it is not uniform and orderly in one unison, and it changes in quality and character.
Similarly we will say regarding the Creator, since the signs of wisdom in His creations are similar and uniform, we must conclude that one Creator created them, because the existence of the created things is impossible without Him.
also in ch.6
There are some people who claim that the world came into being by chance, without a Creator who created it and without a Maker who formed it. It is amazing to me how a rational, healthy human being could entertain such a notion. If such a person heard someone else saying the same thing about a water wheel, which turns to irrigate part of a field or a garden, saying that it came to be without a craftsman who designed it and toiled to assemble it and placed each part for a beneficial purpose - he would be greatly amazed on him, consider him a complete fool, and be swift to call him a liar and reject his words. And since he would reject such a notion for a mere simple, insignificant water wheel, which was made through basic design for rectifying a small piece of land - how could he allow himself to entertain such a notion for the entire universe which surrounds the earth and everything in it, and which is designed with a wisdom that no rational human intellect is capable of fathoming, and which is prepared for the benefit of the earth and everything on it. How could he claim that it came to be without purposeful intent and thought of a capable wise Being?
It is evident to us that for things which come about without the intent of an intender (i.e. an intelligence who designed it with a purpose) - none of them will display any trace of wisdom or ability. Behold and see, that if a man suddenly pours ink on clean paper, it would be impossible for there to be drawn on it orderly writing and legible lines like it would be with a quill, and if a man brought before us orderly writing from what cannot be written without use of a quill, and he would say that ink was spilled on paper, and the form of the writing happened on its own, we would be quick to call him a liar to his face and tell him that it must have been written with an intelligent person's intent
(BTW, nobody has ever refuted this premise. the evidence for evolution is for things like common ancestry but nobody has ever observed a functional complex structure arising from unguided natural processes.)
According to this website, Rabbenu Bachya makes this argument in “The Duties of the Heart,” The Gate of Oneness, Chapter 6:
Do you not realize that if ink were poured out accidentally on a blank sheet of paper, it would be impossible that proper writing should result, legible lines that are written with a pen? Imagine a person bringing a sheet of handwriting that could only have been composed with a pen. He claims that ink spilled on the paper and these written characters had accidentally emerged. We would charge him to his face with falsehood, for we could feel certain that this result could not have happened without an intelligent person’s purpose.
Since this seems impossible in the case of letters whose formation is conventional, how can one assert that something far subtler in its design and which manifests in its fashioning a depth and complexity infinitely beyond our comprehension could have happened without the purpose, power, and wisdom of a wise and mighty designer? (“The Duties of the Heart,” The Gate of Oneness, Chapter 6)
If you don't believe this quote is authentic, you can also see this quote in Rabbi Bleich's With Perfect Faith, which has a Google preview, and The Duties of the Heart itself (which only has snippet view).