Hadith 5:657
'Abd al-A'la narrated to us, from Yunus, from al-Hasan, regarding a man who bought merchandise and Zakat became due upon it. He said: He shall pay Zakat on it according to its value on the day it became due.
حدثنا عبد الأعلى عن يونس عن الحسن في رجل اشترى متاعا فحلت فيه الزكاة فقال يزكيه بقيمته يوم حلت