Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that a mudabbar slave is not to be sold.
This is the authentic version, from Ibn Umar as his own statement (mawquf); it has also been narrated as the Prophet's words (marfu') with a weak chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21571