Language:
كتاب الزكاة
24  
Obligatory Charity Tax (Zakat)
(35)
Chapter: If a property is equally owned by two partners, its Zakat is to be paid as a whole
(35)
باب مَا كَانَ مِنْ خَلِيطَيْنِ فَإِنَّهُمَا يَتَرَاجَعَانِ بَيْنَهُمَا بِالسَّوِيَّةِ
Sahih al-Bukhari 1451

Narrated Anas:

Abu Bakr wrote to me what Allah's Messenger (ﷺ) has made compulsory (regarding Zakat) and this was mentioned in it: If a property is equally owned by two partners, they should pay the combined Zakat and it will be considered that both of them have paid their Zakat equally.

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ، قَالَ حَدَّثَنِي أَبِي قَالَ، حَدَّثَنِي ثُمَامَةُ، أَنَّ أَنَسًا، حَدَّثَهُ أَنَّ أَبَا بَكْرٍ ـ رضى الله عنه ـ كَتَبَ لَهُ الَّتِي فَرَضَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ وَمَا كَانَ مِنْ خَلِيطَيْنِ فَإِنَّهُمَا يَتَرَاجَعَانِ بَيْنَهُمَا بِالسَّوِيَّةِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 1451
In-book reference : Book 24, Hadith 54
USC-MSA web (English) reference : Vol. 2, Book 24, Hadith 531
  (deprecated numbering scheme)