Musannaf Ibn Abi Shayba 13:1385

Abu Bakr narrated to us, he said: Mu'tamir narrated to us, from Layth, from Mujahid, he said: Regarding the *mukātab* (a slave who has entered into a contract of manumission), you give him a quarter of his entire *kitābah* (contractual amount), you hasten it for him from your wealth.