Sahih (Sahih al-Bukhari)
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 ".
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