The Shaykh (al-Bayhaqi) said: Al-Hasan al-Basri (may Allah have mercy on him) said concerning a man upon whom zakah became obligatory but who did not pay it until his wealth was gone: "It is a debt upon him until he pays it."
Mu'adh bin Mu'adh (may Allah have mercy on him) narrated it from Ash'ath (may Allah have mercy on him), from al-Hasan (may Allah have mercy on him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7282