ENOfficial
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.
Source: sunnah.com
Other translations
Arabic
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 صلى الله عليه وسلم قَالَ " وَمَا كَانَ مِنْ خَلِيطَيْنِ فَإِنَّهُمَا يَتَرَاجَعَانِ بَيْنَهُمَا بِالسَّوِيَّةِ ".
sunnah.com