Muwatta Malik, Book 17, Hadith 41 (USC-MSA)

Yahya related to me from Malik that Abdullah ibn Dinar said, "I asked Said ibn al-Musayyab about zakat on work-horses, and he said, 'Is there any zakat on horses ?' "

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، أَنَّهُ قَالَ سَأَلْتُ سَعِيدَ بْنَ الْمُسَيَّبِ عَنْ صَدَقَةِ الْبَرَاذِينِ، فَقَالَ وَهَلْ فِي الْخَيْلِ مِنْ صَدَقَةٍ

sunnah.com

also referenced under: Book 17, Hadith 617 (In-Book)