Sunan an-Nasa'i, Hadith 2470 ⓘ
It was narrated from Abu Hurairah that the Prophet said: "A person does not have to pay Sadaqah on his horse or his slave."
أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا يَحْيَى، عَنْ خُثَيْمٍ، قَالَ حَدَّثَنَا أَبِي، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لَيْسَ عَلَى الْمَرْءِ فِي فَرَسِهِ وَلاَ فِي مَمْلُوكِهِ صَدَقَةٌ " .
also referenced under: Book 23, Hadith 0 (In-Book) & Volume 3, Book 23, Hadith 2472 (DarusSalam)