Musnad Ahmad 30:2617

Waki' narrated to us, he said: Sufyan narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A child shall not repay his father, unless he finds him owned, then he purchases him and sets him free."