A Bedouin once recited the following couplet:

البعرة تدل على البعير و آثار الأقدام على المسير
فسماء ذات أبراج و أرض ذات فجاج كيف لا تدل على اللطيف الخبير 

"When a lump of dung indicates that a camel had passed by and footsteps indicate that a person had passed by, then how could a sky filled with constellations and an earth crossed with paths not indicate the existence of the Creator who is most-gracious and all-aware?"

(Noorul Anwaar page. 173)

An atheist once asked Imaam Shaafi‘ee (rahimahullah) to prove the existence of the creator, Allah Ta‘ala. Imaam Shaafi‘ee (rahimahullah) answered, “Look at the leaves of the mulberry tree. The color, taste, smell, composition and properties of every leaf are the same. Despite being exactly the same, when consumed by the silk worm, silk is produced. When visited by the bee, honey is produced. When consumed by the goat, dung is produced and when consumed by the musk deer, musk is produced. Only the design of a creator who is eternal and all powerful could cause so many diverse things to be produced from one substance. Otherwise, logic would demand that the end product of all be the same as the substance which entered all was the same.”
(‘Aqaa’id-ul-Islaam page.42)

A person once came to Imaam Maalik (rahimahullah) and asked him to present proof for the existence of a creator. Imaam Maalik (rahimahullah) answered, “Look at the face of a person. As small as it may be, every person’s face comprises of eyes, a nose, ears, a tongue, cheeks, gums etc. Despite every face containing the exact same organs, there are no two faces that are exactly the same in appearance and shape. Voices are different, tones are different and limbs are different. This uniqueness with which every person is blessed can only be the result of a creator’s design.”
(‘Aqaa’id-ul-Islaam page.42)

A group of atheists once approached Imaam Abu Hanifah (rahimahullah) with the sinister intention of murdering him. On noticing them, Imaam Abu Hanifah (rahimahullah) decided to engage them in dialogue and so asked them, “If a person had to claim that he saw a ship which had neither a captain nor sailors to steer and navigate it, sailing through the waves of the ocean in a perfectly straight line, loading goods on one shore and offloading them on another shore, all on its own without a person to oversee and control it, what would you say?” The group immediately exclaimed, “Such a phenomenon is so illogical that no intelligent person would accept is as possible!” Imaam Abu Hanifah (rahimahullah) responded saying, “What has then happened to your intelligence? When you accept that one single ship cannot sail and operate without sailors aboard, then how do you accept that the entire universe can function and operate without Allah to control it?” The entire group was astonished at this line of reasoning and immediately repented and accepted Islam at the hands of Imaam Abu Hanifah (rahimahullah).

(‘Aqaa’id-ul-Islaam page.41)