The story goes that the Caliph Abu Ja’far al-Mansoor (709-775c) had a memory so good he was able to memorize any poem after hearing it just once. So he announced a competition challenging all the poets in his empire to come up with a new poem which he never heard before. A boy was placed behind a curtain in the Caliph’s court who was known for his ability to memorize any poem he heard so long as it was recited twice to him. The Caliph also placed behind the curtain a slave girl who was know for her remarkable ability to memorize any poem she heard so long as it was recited three times. When the day of the competition arrived, one by one poets from all around the empire flocked to the Caliph’s Court to try and claim the prize (which was an amount of gold equivalant in weight to the piece of paper the poem was written in) reserved for the one who could recite a poem the Caliph never heard before.

The first poet to enter the Caliph's court was absolutely sure he would win the prize money. After all, he spent all night composing a new poem which of course he was sure no one has ever heard before. So when it came time to recite the poem in front of the Caliph the poet smiled to himself, thinking the prize would surely be his. The poet began to recite his new poem. As he continued reciting his heart filled with glee just thinking of what he would do with the prize . When the poet finished reciting, he stepped forward fully expecting to receive the prize money from the Caliphate for accomplishing his task. To his shock the Caliphate responded nonchalantly, “I’ve heard that poem before.” The Caliph began to recite the poem line for line without making a single mistake. When he finished the poet was in an utter state of shock. “How can this be? I spent all night composing a new poem and the Ameer al-Mu'mineen claims he heard it before?” the poet asked. The Caliph responding to the poet’s bewilderment said, “You look surprised. I assure you that I have heard this poem before. In fact I know of others who have heard it as well. Bring the boy!” Upon hearing the Caliph's command the royal guards brought the boy who was hiding behind the curtain. “Have you heard this poem before?” the Caliph asked the boy. “Yes” the boy responded. Then the boy began to recite the poem and completed it without making a single mistake. Of course the boy’s extraordinary talent of memorizing a poem after hearing it only twice (the first recitation being the poet’s and the second being the Caliphate’s) was unknown to the poet. Again the poet was beside himself in shock. Unable to comprehend how both the Caliph and the boy claimed to have heard and memorized the poem he spent all night writing. Not satisfied with poet’s current state of shock the Caliph said to the poet, “There are others who have heard this poem as well. Bring out the slave girl!” Upon the Caliph’s command the royal guard brought out the slave girl from behind the same curtain the boy was hiding behind. The Caliph asked the slave girl, “Have you heard this poem before?” “Yes” replied the slave girl. Then the slave girl began to recite the poem and completed it without making a single mistake. Of course the slave girl’s extraordinary talent of memorizing a poem after hearing it only three times (the first recitation being the poet’s, the second being the Caliph’s, and the third being the boy’s) was unknown to the poet. Unable to bear the perplexities of the situation the poet left the Caliph’s court without achieving his goal of claiming the prize.

The Caliphcontinued playing this trick on all the poets who came to his court. One by one poets from all over the Caliphate entered the Caliph’s court hoping to receive the prize money he was offering by reciting a new poem. Apparently so many poets came and failed that they started gathering outside of the Caliph’s court. Each of them was sure that the poems they recited in the presence of the Caliph was new and there was no way possible that the Caliph, the boy, and the slave girl all could have heard it before and have memorized it before. Bewildered they sat amongst themselves discussing the situation.

Seeing the gathering of poets outside the Caliphate’s court Al-Asma'i, the famous poet, stopped and inquired as to their situation. After hearing their story Al-Asma'i knew the trick the Caliph was playing on the poets. Al-Asma'i entered upon the Caliph, and after taking permission he began to recite his poem.

صـوت صــفير الـبلبـلي *** هيج قـــلبي الثمــلي

المـــــــاء والزهر معا *** مــــع زهرِ لحظِ المٌقَلي

و أنت يا ســـــــــيدَ لي *** وســــــيدي ومولي لي

فكــــــــم فكــــم تيمني *** غُـــزَيلٌ عقــــــــــيقَلي

قطَّفتَه من وجــــــــــنَةٍ *** من لثم ورد الخــــجلي

فـــــــقال لا لا لا لا لا *** وقــــــــد غدا مهرولي

والخُـــــوذ مالت طربا *** من فعل هـــذا الرجلي

فــــــــولولت وولولت *** ولـــــي ولي يا ويل لي

فقلت لا تولولـــــــــي *** وبيني اللؤلؤ لــــــــــي

قالت له حين كـــــــذا *** انهض وجــــــد بالنقلي

وفتية سقــــــــــــونني *** قـــــــــهوة كالعسل لي

شممـــــــــــتها بأنافي *** أزكـــــــى من القرنفلي

في وســط بستان حلي *** بالزهر والســـــرور لي

والعـــود دندن دنا لي *** والطبل طبطب طب لـي

طب طبطب طب طبطب *** طب طبطب طبطب طب لي

والسقف سق سق سق لي *** والرقص قد طاب لي

شـوى شـوى وشــــاهش *** على ورق ســـفرجلي

وغرد القمري يصـــــيح *** ملل فـــــــــــي مللي

ولــــــــــــو تراني راكبا *** علــــى حمار اهزلي

يمشي علــــــــــــى ثلاثة *** كمـــــشية العرنجلي

والناس ترجــــــــم جملي *** في الســوق بالقلقللي

والكـــــــــل كعكع كعِكَع *** خلفي ومـــن حويللي

لكـــــــــــن مشيت هاربا *** من خشـــية العقنقلي

إلى لقاء مــــــــــــــــلك *** مــــــــــعظم مبجلي

يأمر لي بخـــــــــــــلعة *** حمـــراء كالدم دملي

اجــــــــــــر فيها ماشيا *** مبغــــــــــددا للذيلي

انا الأديب الألمــعي من *** حي ارض الموصلي

نظمت قطــــعا زخرفت *** يعجز عنها الأدبو لي

أقول في مطلعــــــــــها *** صوت صفير البلبلي

The Caliph was in an utter state of shock. He had never heard a poem like that before. The poem was filled with onomatopoeia & tongue twisters making it impossible to memorize. The Caliph called for the boy and the slave girl, and asked them both, “Have you heard anything like that poem before?” They replied, “No!” The Caliphate then ordered that al-Asma'i bring his paper so that it can be weighed. Al-Asma'i said: "O Ameer al-Mu'mineenbut I haven't written it on paper, I have written it on marble, which can only be carried by four of your guards!" It was weighed and ended up weighing too much, al-Asma'i said that he would leave the gold on one condition, that the Caliph should give gold everytime a poet comes with a new poem, because they too have families & dependants and the Caliph had cut them off, and obviously the Caliph agreed.

Source (I edited a bit)