Musannaf Ibn Abi Shayba 5:44

Abu Usama narrated to us, from Hisham, from Ibn Sirin, from Khalid al-Hadhdha’—and he was a slave to the Banu Mujashi’—he said: I said to Ibn Umar, "Is there Zakat upon the slave?" He said, "Is he a Muslim?" I said, "Yes." He said, "Upon him, in every two hundred dirhams, is five dirhams."