It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "There is no zakah on horses and slaves, except Zakat al-Fitr on slaves."
This is the more correct version. Ubaydullah's narration of it from Abu al-Zinad is not mahfuz (preserved), and Makhul did not hear it from Irak; he only narrated it from Sulayman bin Yasar, from Irak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7402