Musannaf Ibn Abi Shayba 13:1454

Abu Bakr narrated to us, he said: Abbad bin al-Awwam narrated to us, from Hajjaj, from Husayn, from al-Sha’bi, from al-Harith, from Ali, who said: If two installments (nujuman) of the mukatab (slave under a contract of manumission) follow one another, and he enters into the year without having paid his installments, he is returned to slavery.