Sahih al-Bukhari
Obligatory Charity Tax (Zakat)
كتاب الزكاة
Chapter 77: Sadaqat-ul-Fitr on a slave as well as on a free Muslim
صَدَقَةِ الْفِطْرِ عَلَى الْحُرِّ وَالْمَمْلُوكِ
And Az-Zuhri says that it is also compulsory on the slaves for sale; both the prescribed Zakat on wealth as well as Sadaqat-ul-Fitr are to be paid.
Ibn `Umar said, "The Prophet (ﷺ) made incumbent on every male or female, free man or slave, the payment of one Sa' of dates or barley as Sadaqat-ul-Fitr (or said Sadaqa-Ramadan)." The people then substituted half Sa' of wheat for that. Ibn `Umar used to give dates (as Sadaqat-ul-Fitr). Once there was scarcity of dates in Medina and Ibn `Umar gave barley. 'And Ibn `Umar used to give Sadaqat-ul- Fitr for every young and old person. He even used to give on behalf of my children. Ibn `Umar used to give Sadaqat-ul-Fitr to those who had been officially appointed for its collection. People used to give Sadaqat-ul-Fitr (even) a day or two before the `Id.
Chapter 78: Sadaqat-ul-Fitr is obligatory on the young and the old
صَدَقَةِ الْفِطْرِ عَلَى الصَّغِيرِ وَالْكَبِيرِ
Allah's Messenger (ﷺ) has made Sadaqat-ul-Fitr obligatory, (and it was), either one Sa' of barley or one Sa' of dates (and its payment was obligatory) on young and old people, and on free men as well as on slaves.