Sahih Bukhari Volume 2, Book 24, Hadith Number 528.

Narated By Anas : Abu Bakr wrote to me what Allah had instructed His Apostle (p.b.u.h) to do regarding the one who had to pay one Bint Makhad (i.e. one year-old she-camel) as Zakat, and he did not have it but had got Bint Labun (two year old she-camel). (He wrote that) it could be accepted from him as Zakat, and the collector of Zakat would return him 20 Dirhams or two sheep; and if the Zakat payer had not a Bint Makhad, but he had Ibn Labun (a two year old he-camel) then it could be accepted as his Zakat, but he would not be paid anything.