A Sura by Musaylima

   ... Then he began to utter rhymes in saj` and speak in 
   imitation of the style of the Quran: 

   "God has been gracious to the pregnant woman; 
   He has brought forth from her a living being that can move; 
   from her very midst."

Quoted from Sirat Ar-Rasool by Ibn Ishaq, English translation from The Life of Muhammad by A. Guillaume, page 637.

Interestingly, Musaylima is mentioned in several passags in Ibn Ishaq's Sirat, and he is rejected because he was (supposedly) a liar. There is no mention that the people around him rejected his claims because his sura was linguistically inferior. In any case, this is listed here not so much as a serious challenge but because it is historically an important case and one of the very few that survived from the early time of Islam. Most other attempts have been destroyed by the Muslims.

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