Abu Dawud Book 3, Hadith Number 1578.
Narated By Ubayy ibn Ka’b : The Apostle of Allah (PBUH) commissioned me as a collector of zakat. I visited a man. When he had collected his property of camels, I found that a she-camel in her second year was due from him.
I said to him: Pay a she-camel in her second year, for she is to be paid as sadaqah (zakat) by you.
He said: That one is not worthy of milking and riding. Here is another she-camel which is young, grand and fat. So take it.
I said to him: I shall not take an animal for which I have not been commanded. The Apostle of Allah (PBUH) is here near to you. If you like, go to him, and present to him what you presented to me. Do that; if he accepts it from you, I shall accept it; if he rejects it, I shall reject it.
He said: I shall do it. He accompanied me and took with him the she-camel which he had presented to me. We came to the Apostle of Allah (PBUH). He said to him: Prophet of Allah, your messenger came to me to collect zakat on my property. By Allah, neither the Apostle of Allah nor his messenger has ever seen my property before. I gathered my property (camels), and he estimated that a she-camel in her second year would be payable by me. But that has neither milk nor is it worth riding. So I presented to him a grand young she-camel for acceptance as zakat. But he has refused to take her. Look, she is here; I have brought her to you, Apostle of Allah. Take her.
The Apostle of Allah (PBUH) said: That is what is due from you. If you give voluntarily a better (animal) Allah will give a reward to you for it. We accept her from you.
She is here, Apostle of Allah; I have brought her to you. So take her. The Apostle of Allah (PBUH) then ordered me to take possession of it, and he prayed for a blessing on his property.