Sahih Bukhari Volume 2, Book 24, Hadith 1449

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ، قَالَ حَدَّثَنِي أَبِي قَالَ، حَدَّثَنِي ثُمَامَةُ، أَنَّ أَنَسًا ـ رضى الله عنه ـ حَدَّثَهُ أَنَّ أَبَا بَكْرٍ ـ رضى الله عنه ـ كَتَبَ لَهُ الَّتِي أَمَرَ اللَّهُ رَسُولَهُ صلى الله عليه وسلم ‏"‏ وَمَنْ بَلَغَتْ صَدَقَتُهُ بِنْتَ مَخَاضٍ وَلَيْسَتْ عِنْدَهُ وَعِنْدَهُ بِنْتُ لَبُونٍ فَإِنَّهَا تُقْبَلُ مِنْهُ، وَيُعْطِيهِ الْمُصَدِّقُ عِشْرِينَ دِرْهَمًا أَوْ شَاتَيْنِ، فَإِنْ لَمْ يَكُنْ عِنْدَهُ بِنْتُ مَخَاضٍ عَلَى وَجْهِهَا، وَعِنْدَهُ ابْنُ لَبُونٍ فَإِنَّهُ يُقْبَلُ مِنْهُ وَلَيْسَ مَعَهُ شَىْءٌ ‏"

Narrated 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.