He called a slave of his and freed him, then said, "I have no more reward from it than this," picking up something tiny from the ground. "I heard the Messenger of Allah ﷺ say, 'Whoever slaps his slave, his expiation is to free him.'"
حدثنا وكيع عن سفيان عن فراس عن أبي صالح عن زاذان
أنه دعا غلاما له فأعتقه فقال ما لي من أجره مثل هذا لشيء رفعه من الأرض سمعت رسول الله ﷺ يقول من لطم غلامه فكفارته عتقه
Its isnad is sahih. Firas is Ibn Yahya Hamdani Harithi, previously mentioned in 4333. Abu Salih is Zakwan Samman. Muslim narrated the hadith in 2:19 through the route of Abu Awana, Shubah, and Sufyan Thawri, all of them from Firas.