Musannaf Ibn Abi Shayba 13:2266

Abu Bakr narrated to us, he said: Waki’ narrated to us, from al-Rabi’, from al-Hasan and Ibn Sirin, that they both disliked for the *mukātab* (a slave under a contract of manumission) to say: "Hasten [payment] for me, and I will remit [a portion] for you."