Zazan reported,

Ibn Umar freed a slave of his and said, "I will not get even this much reward for it," picking up something from the ground, "the weight of this, or the like of this. I heard the Messenger of Allah ﷺ say, 'Whoever slaps his slave or strikes him, his expiation is to set him free.'"

حدثنا وكيع عن سفيان عن فراس عن أبي صالح

عن زاذان

أن ابن عمر أعتق عبدا له فقال ما لي من أجره وتناول شيئا من الأرض ما يزن هذه أو مثل هذه سمعت رسول الله ﷺ يقول من لطم غلامه أو ضربه فكفارته عتقه