Maliks Muwatta Book 17, Hadith Number 52.
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to pay the zakat al-fitr for those slaves of his that were at Wadi’l-Qura and Khaybar.
Yahya related to me that Malik said, “The best that I have heard about the zakat al-fitr is that a man has to pay for every person that he is responsible for supporting and whom he must support. He has to pay for all his mukatabs, his mudabbars, and his ordinary slaves, whether they are present or absent, as long as they are muslim, and whether or not they are for trade. However, he does not have to pay zakat on any of them that are not muslim.”
Malik said, concerning a runaway slave, “I think that his master should pay the zakat for him whether or not he knows where he is, if it has not been long since the slave ran away and his master hopes that he is still alive and will return. If it has been a long time since he ran away and his master has despaired of him returning then I do not think that he should pay zakat for him.’
Malik said, “The zakat al-fitr has to be paid by people living in the desert (i.e. nomadic people) just as it has to be paid by people living in villages (i.e. settled people), because the Messenger of Allah, may Allah bless him and grant him peace, made the zakat al-fitr at the end of Ramadan obligatory on every muslim, whether freeman or slave, male or female.”