Musaylimah al Kadhdhab’s Response to the Challenge:

Musaylimah Al-Kadhdhab (Musaylimah the Liar) was a man who claimed to be a Prophet during the Prophet Muhammad (peace be upon him's) lifetime.

Here is an example of his 'Quran' which he made up, mentioned in Tafseer Ibn Katheer (tafseer of surah al 'Asr).
 

ذكروا أن عمرو بن العاص وفد على مسيلمة الكذاب [ لعنه الله ] وذلك بعد ما بعث رسول الله صلى الله عليه وسلم وقبل أن يسلم عمرو فقال له مسيلمة : ماذا

أنزل على صاحبكم في هذه المدة ؟ قال لقد أنزل عليه سورة وجيزة بليغة . فقال : وما هي ؟ فقال : " والعصر إن الإنسان لفي خسر إلا الذين آمنوا وعملوا

الصالحات وتواصوا بالحق وتواصوا بالصبر " ففكر مسيلمة هنيهة ثم قال : وقد أنزل علي مثلها . فقال له عمرو : وما هو ؟ فقال : يا وبر يا وبر ، إنما أنت

أذنان وصدر ، وسائرك حفز نقز . ثم قال : كيف ترى يا عمرو ؟ فقال له عمرو : والله إنك لتعلم أني أعلم أنك تكذب .

 

http://www.islamweb.net/newlibrary/display_book.php?idfrom=1976&idto=1976&bk_no=49&ID=2072

They have mentioned that `Amr bin Al-`As went to visit Musaylimah Al-Kadhdhab after the Messenger of Allah was commissioned (as a Prophet) and before `Amr had accepted Islam. Upon his arrival, Musaylimah said to him,

"What has been revealed to your friend (Muhammad) during this time''

`Amr said, "A short and concise Surah has been revealed to him.''

Musaylimah then said, "What is it'' `Amr replied;

وَالْعَصْرِ – إِنَّ الإِنسَـنَ لَفِى خُسْرٍ – إِلاَّ الَّذِينَ ءَامَنُواْ وَعَمِلُواْ الصَّـلِحَـتِ وَتَوَاصَوْاْ بِالْحَقِّ وَتَوَاصَوْاْ بِالصَّبْ

(By Al-`Asr. Verily, man is in loss. Except those who believe and do righteous deeds, and recommend one another to the truth, and recommend one another to patience.)

So Musaylimah thought for a while. Then he said, "Indeed something similar has also been revealed to me.''

`Amr asked him, "What is it''

He replied, "O Wabr* (a small, furry mammal; Hyrax), O Wabr! You are only two ears and a chest, and the rest of you is digging and burrowing.''

Then he said, "What do you think, O `Amr''

So `Amr said to him, "By Allah! Verily, you know that I know you are lying.''

The Wabr is a small animal that resembles a cat, and the largest thing on it is its ears and its torso, while the rest of it is ugly. Musaylimah intended by the composition of these nonsensical verses to produce something which would oppose the Qur'an. Yet, it was not even convincing to the idol worshipper of that time!

(From Tafsir Ibn Kathir, Surat al-Asr)
http://tafsir.com/default.asp?sid=103&tid=59151