Abu Dawud Book 3, Hadith Number 1576.
Narated By Sa’r ibn Disam : Muslim ibn Shu’bah said: Nafi’ ibn Alqamah appointed my father as charge d’affaires of his tribe, and commanded him to collect sadaqah (zakat) from them. My father sent me to a group of them; so I came to an aged man called Sa’r ibn Disam
I said: My father has sent me to you to collect zakat from you. He asked: What kind of animals will you take, my nephew? I replied: We shall select the sheep and examine their udders. He said: My nephew, I shall narrate a tradition to you. I lived on one of these steppes during the time of the Apostle of Allah (PBUH) along with my sheep. Two people riding a camel came to me.
They said to me: We are messengers of the Apostle of Allah (PBUH), sent to you so that you may pay the sadaqah (zakat) on your sheep.
I asked: What is due from me for them?
They said: One goat. I went to a goat which I knew was full of milk and fat, and I brought it to them.
They said: This is a pregnant goat. The Apostle of Allah (PBUH) prohibited us to accept a pregnant goat.
I asked: What will you take then? They said: A goat in its second year or a goat in its third year. I then went to a goat which had not given birth to any kid, but it was going to do so. I brought it to them.
They said: Give it to us. They took it on the camel and went away.