Al-Hasan (may Allah have mercy on him) narrates from the Prophet (peace and blessings be upon him) in mursal form: "Whoever pays the zakah of his wealth has fulfilled the right that was due from him, and whoever gives more, that will be a source of merit for him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7241