(35) باب مَا كَانَ مِنْ خَلِيطَيْنِ فَإِنَّهُمَا يَتَرَاجَعَانِ بَيْنَهُمَا بِالسَّوِيَّةِ
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.
Reference: Sahih al-Bukhari 1451
In-book reference: Book 24, Hadith 55
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 24, Hadith 531