Previous prophets performed miracles to prove that their message was not their own but God's message. In the case of the Qur'an, the message itself is the miracle.
The prophet Muhammad, on whom be peace, did not know how to read or write. Yet the Qur'an which was being revealed to him was (and still is) the highest pinnacle of literary beauty and excellence in the Arabic Language. It was beyond the ability of the prophet to produce this book. And God declares that it is beyond the ability of all of humankind and spirit-kind to produce a book like the Qur'an.
The Qur'an was revealed piece by piece over a period of twenty-three years. During this time, God challenged humankind four times to produce a book like the Qur'an, and they couldn't do it. When the prophet and his followers were being persecuted in Mecca, God showed the persecutors a simple way by which they can wipe out the message of the Qur'an. God stated that this book cannot be produced by men or spirit creatures. Obviously, if they pooled together their abilities and composed a book like the Qur'an they would have proved the Qur'an to be false in its claim. But they couldn't. And here is what the Qur'an claims:
Say: If the whole of mankind and Jinns were to gather together to produce the like of this Qur'an they could not produce the like thereof, even if they backed up each other with help and support (Qur'an 17:88).
Although people ardently wished to block the message of the Qur'an and they tried every thing they could to stifle it, yet they did not try this obvious strategy. Why? Because it cannot be done. So God reduced the challenge for them: never mind the whole book; produce just ten chapters if you can. This challenge is mentioned in Surah 11:13 in the Qur'an. Again they couldn't do it. And they were the literary giants of their day. They held fairs to promote their own literary masterpieces. They hung their prized compositions on the door of the house of pilgrimage for all to see. But they could not match the stunning verses of the Qur'an.
God reduced the challenge further still (see Qur'an 10:38). If they cannot produce ten chapters then how about one chapter? Just one! And they couldn't do it, so they eventually pulled down their proud exhibits from where they were hung. The Qur'an had proved its point: this is no man-made book.
Yet they tried everything to banish the Qur'an from the face of the earth. They even made an attempt on the life of the prophet. He fled to Madinah, another city. And there the Qur'an kept coming to him piece by piece. The challenge was repeated yet again. This time God reduced the emphasis as if to say, okay you couldn't produce a chapter like it; now produce a chapter even remotely like
it. This challenge is mentioned in the Qur'an in chapter 2, verse 23. They failed again. And the world has failed since. The challenge is still there in the Qur'an, and everyone who disbelieves the Qur'an can still pool together their abilities and resources to try and produce one like it. But they cannot. Isn't this an awesome miracle?