Sahih Bukhari Volume 009, Book 086, Hadith Number 090t.

Narrated By Ibn Abbas : Sa'd bin 'Ubada Al-Ansari sought the verdict of Allah's Apostle regarding a vow made by his mother who had died before fulfilling it. Allah's Apostle said, "Fulfill it on her behalf." Some people said, "If the number of camels reaches twenty, then their owner has to pay four sheep as Zakat; and if their owner gives them as a gift or sells them in order to escape the payment of Zakat cunningly before the completion of a year, then he is not to pay anything, and if he slaughters them and then dies, then no Zakat is to be taken from his property."