Sahih Al Bukhari Hadith Number 1448

Narrated Anas: bu Bakr wrote to me what Allah had instructed His Apostle (peace be upon him) 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 .

24  Obligatory Charity Tax (Zakat)
Hadith number 1448
Volume no 2
Chapter: 33 (Zakat may be paid in kind)