The Messenger of Allah ﷺ said, "A child cannot repay a parent unless he finds him enslaved, then buys him and sets him free."
قال رسول الله ﷺ: لا يجزي ولد والدا إلا أن يجده مملوكا، فيشتريه، فيعتقه.
Muslim narrated it in the chapter on manumission (1510), through the route of Jareer, from Suhayl, from his father, from Abu Hurairah, who related it as mentioned.As for his saying, "he buys him and thereby frees him," this means that the manumission takes effect through the purchase itself, without needing a separate act of manumission. A separate act of manumission would imply that he could keep him under his ownership if he wished. But none of the scholars have said that a father remains owned by his son until the son formally frees him, because Allah Almighty says, "Your Lord has decreed that you worship none but Him, and that you show kindness to your parents" [Isra 17:23]. Allah Almighty has thus placed worship of Him and kindness to one's parents on equal footing in terms of obligation, and it is not an act of kindness to keep one's father in one's ownership.