Al-Za'farani narrated this from Imam al-Shafi'i (may Allah have mercy on him); then he retracted it in his new position, and his retraction is more fitting, because of the reports that have passed and other apparent proofs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / Q7627
Uthman bin al-Aswad (may Allah have mercy on him) said that he heard Ata (may Allah have mercy on him) say: There is no zakah upon you on a debt owed to you, even if it is with solvent people.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7627