Abū Nuwās asked Khalaf for permission to compose poetry, ...

Abū Nuwās asked Khalaf for permission to compose poetry, and Khalaf said: ‘I refuse to let you make a poem until you memorize a thousand passages of ancient poetry, including chants, odes, and occasional lines’. So Abū Nuwās disappeared; and after a good long while, he came back and said, ‘I’ve done it’.

‘Recite them’, said Khalaf.

So Abū Nuwās began, and got through the bulk of the verses over a period of several days. Then he asked again for permission to compose poetry. Said Khalaf, ‘I refuse, unless you forget all one thousand lines as completely as if you had never learned them’.

‘That’s too difficult’, said Abū Nuwās. ‘I’ve memorized them quite thoroughly!’

‘I refuse to let you compose until you forget them’, said Khalaf.

So Abū Nuwās disappeared into a monastery and remained in solitude for a period of time until he forgot the lines. He went back to Khalaf and said, ‘I’ve forgotten them so thoroughly that it’s as if I never memorized anything at all’.

Khalaf then said, ‘Now go compose!’

Ibn Manẓūr, in Tales of Abū Nuwās, via Heller-Roazen’s Echolalias.