Narrated Anas:

that Abu Bakr As-Siddiq wrote to him the law of Zakat which was made obligatory by Allah's Apostle. He wrote: 'Partners possessing joint property (sheep) have to pay its Zakat equally.

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ الْمُثَنَّى، قَالَ حَدَّثَنِي أَبِي قَالَ، حَدَّثَنِي ثُمَامَةُ بْنُ عَبْدِ اللَّهِ بْنِ أَنَسٍ، أَنَّ أَنَسًا، حَدَّثَهُ أَنَّ أَبَا بَكْرٍ ـ رضى الله عنه ـ كَتَبَ لَهُ فَرِيضَةَ الصَّدَقَةِ الَّتِي فَرَضَ رَسُولُ اللَّهِ ﷺ قَالَ
" وَمَا كَانَ مِنْ خَلِيطَيْنِ فَإِنَّهُمَا يَتَرَاجَعَانِ بَيْنَهُمَا بِالسَّوِيَّةِ ".