Musannaf Ibn Abi Shayba 13:2264

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Zakariya, from al-Sha’bi, regarding a man who says to his mukatab (slave under a contract of manumission): "I will remit [a portion of the debt] for you, provided you hasten [the payment of the remainder] for me." He disliked it.