Al-Hasan (may Allah have mercy on him) narrates from Sayyiduna Zayd bin Thabit (may Allah be pleased with him) that a mudabbar slave is not to be sold.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21570
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
Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "The mudabbar slave is not to be sold or given as a gift, and he is freed out of the third (of the estate)."
Ali (al-Daraqutni) said: No one but Ubaydah bin Hassan narrated it as connected to the Prophet, and he is da'if (weak); it is only from Ibn Umar as his own statement (mawquf), and it is not established as marfu'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21572