The Messenger of Allah ﷺ said, "A child cannot repay his parent unless he finds him enslaved, then buys him and sets him free."
حدثنا أبو بكر بن أبي شيبة حدثنا جرير عن سهيل عن أبيه
قال رسول الله ﷺ لا يجزي ولد والدا إلا أن يجده مملوكا فيشتريه فيعتقه
Muslim narrated it in the Book of Manumission, chapter on the virtue of freeing one's parent, hadith 1510, from Ibn Abu Shayba with the same wording, and it is in Musannaf, 8/351.