It is narrated from Abdullah bin Dinar (may Allah have mercy on him) who says: I asked Sa'id bin al-Musayyib (may Allah have mercy on him) about the zakat on non-Arab horses, so he said: "Is zakat obligatory on horses?"
Hadith Referenceمسند الإمام الشافعي / كتاب الزكاة / 709
Hadith Takhrijاسناده صحیح اخرجه البيهقي : 4 119 ۔ وفى المعرفة السنن والآثار له (2298) وابن ابي شيبة (10145) ۔ (10146)۔