Volume 3 · Book 44 · Hadith 667
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.
— Sahih Bukhari, Volume 3, Book 44: Partnership, Hadith #667