Abu Dawud Book 15, Hadith Number 3308.
Narated By Maymunah, daughter of Kardam : I went out with my father to see the hajj performed by the Apostle of Allah (PBUH). I saw the Apostle of Allah (PBUH). I fixed my eyes on him. My father came near him while he was riding his she-camel. He had a whip like the whip of scribes. I heard the bedouin and the people say: The whip, the whip. My father came near him and held his foot. She said: He admitted his Prophethood and stood and listened to him.
He said: Apostle of Allah, I have made a vow that if a son is born to me, I shall slaughter a number of sheep at the end of Buwanah in the dale of hill.
The narrator said: I do not know (for certain) that she said: Fifty (sheep).
The Apostle of Allah (PBUH) said: Does it contain any idol?
He said: No. Then he said: Fulfil your vow that you have taken for Allah. He then gathered them (i.e. the sheep) and began to slaughter them. A sheep ran away from them.
He searched for it saying: O Allah, fulfil my vow on my behalf. So he succeeded (in finding it) and slaughtered it.