Language:
كتاب الزكاة
24  
Obligatory Charity Tax (Zakat)
(34)
Chapter: The individual property of different people should neither be gathered together nor the joint property should be divided, in assessing the Zakat
(34)
باب لاَ يُجْمَعُ بَيْنَ مُتَفَرِّقٍ وَلاَ يُفَرَّقُ بَيْنَ مُجْتَمِعٍ
Sahih al-Bukhari 1450

Narrated Anas:

Abu Bakr wrote to me what was made compulsory by Allah's Messenger (ﷺ) 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.

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ الأَنْصَارِيُّ، 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 فَرَضَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ وَلاَ يُجْمَعُ بَيْنَ مُتَفَرِّقٍ، وَلاَ يُفَرَّقُ بَيْنَ مُجْتَمِعٍ، خَشْيَةَ الصَّدَقَةِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 1450
In-book reference : Book 24, Hadith 53
USC-MSA web (English) reference : Vol. 2, Book 24, Hadith 530
  (deprecated numbering scheme)