Jami` at-Tirmidhi 1260

Narrated 'Amr bin Shu'aib: From his father, from his grandfather that he heard the Messenger of Allah (ﷺ) delivering a Khutbah in which he said: "Whoever gives a writ of emancipation to his slave, for one hundred Uqiyyah, and he pays it to him less then ten Uqiyah." - or he said: "Ten Dirham" - "then he becomes incapable (of paying the remainder), the he remains a slave." [Abu 'Eisa said:] This Hadith is Hasan Gharib. And this acted upon according to most of the people of knowledge among the Companions of the Prophet (ﷺ) and others: The Mukatab is a slave as long as something remains due from him for his Kitabah. Al-Hajjaj bin Artat reported similarly from 'Amr bin Shu'aib