Sahih Al Bukhari Hadith Number 1450

Narrated Anas: Abu Bakr wrote to me what was made compulsory by Allah’s Messenger (peace be upon him) and that was (regarding the payments of Zakat): Neither the property of different people may be taken together nor the joint property may be split for fear of (paying more, or receiving less) Zakat.

24  Obligatory Charity Tax (Zakat)
Hadith number 1450
Volume no 2
Chapter: 34 (The individual property of different people should neither be gathered together nor the joint property should be divided, in assessing the Zakat)